Every saga has 4 levels and you can carry a hero and 3 items of your choice to a next mission. You want to raise a hero as your main throughout a saga and try to find most useful and powerful items. Saga maps have possible reachable hero level as 2 level per map. You can get the maximum allowed hero level in a given map by usually clearing most of that map. If you still need exp when you cleared all the naturals and the capturable towns and you can not take down the enemy capital, you can farm the heroes that your enemy sends on a regular basis.

There are permanent stat boosting potions scattered around different maps. Some of them can be found in another level of a saga but I would not rely on it too much and would try to find all of em as possible. They are not game changers on their own but when you get all the different ones you can, they make a difference.

The game can feel like unfair when it comes to hit/miss chance, if you are unhappy with the rolls in a battle, just do not feel bad loading a previous save.

WL’s 15% heal per turn is probably the best ability of the lords. ML’s ability to cast spells twice per turn or having cheaper research costs are limited by your available mana pool. Mana is gained by capturing mana nodes on campaign maps. There is no other way to influence mana gain so your casting power is confined by how fast you can get the nodes on a map and also how many nodes are there on a given map. Since campaign is story driven, you usually do not find many opportunities to blitz the mana nodes. Due to that and limited amount of nodes on a map, a ML is harder to pull than a WL.

Exceptions of this;
-The Empire has the only healer troops available to all the factions and they are pretty potent (due to low initiative of the healers, (20) they cast at the end of each round, providing you one last heal when the fight finishes and especially the mass healing healers’ potential). Also they have “heal” and “haste” spells at level 2 and “major heal” at level 5. On top of these, Empire troops have low hp pools makes heal spells and health potions more effective (since they heal a fixed amount instead of a percentage) compared to high life pool factions like Dwarf and Legion. These perks make a ML as viable as a WL for the Empire.

-The Mountain Clans have the slowest moving heroes and they don not have a flying hero make it very painful to traverse the maps. Water bodies and forests slow down the movement of walking heroes to a crawl. Fortunately, Clans have some spells at their disposal which helps immensely overcoming the traveling issues. “Forestwalking” at level 1 and “seafaring” at level 2 makes heroes of the faction to negate the penalties of those terrain types. On top of that “chant of haste” level 3 spell provides the biggest movement speed boost on the campaign maps, making heroes move twice per turn for their maximum movement points.
Clans having level 2 “healing” spell is not as good as it is for The Empire because of dwarfs lower initiative makes them get hurt first. This combined with large health pools of their units makes percent based healing far more useful but its still good to have.
For this faction MLs are viable almost as WLs because of being able cast movement spells twice per turn combined with some healing here and there.

I completed all 3 campaigns at hard difficulty with WLs besides of Dwarven one which I used a ML.

(There are thiefs but they do not get Exp and so can not carry the game.)
*acronyms

I am not going to discuss the viability of maining the rod planters. They start with 1 leadership which limits them a lot but legion and undead ones have disables as their default attacks which can help you to take down capitals and if you can raise you leadership to a meaningful level to carry some real damage dealers with yourself etc. So in this section evaluations will be about the other 3 hero types.

Scout type heroes start with the highest movement point pools for any faction but flying heroes have the advantage of flying; forests and water bodies (including rivers) applies a movement penalty to the walking heroes. For this reason flying heroes have an advantage over the stats wise faster scout heroes. Besides of Clans, all warrior type heroes are flyers. Legion has their flying scout which is great but all of their heroes can fly so its not a deciding factor for them. Also Clans has no flying hero.

As a general rule, caster type heroes have the biggest damage potential of all. To give an example: If there are 3 enemy units in a battle, “warrior type hero” hitting one unit with 50 damage per turn compared to “caster type” hitting 3 units with 30 per unit for 90 damage makes casters far superior bombers. If you put the 6 possible enemy unit in to the equation, wizard jumps to the 6x30=180 damage per turn damage potential. But this is not the case usually due to 6 unit enemy armies are very rare and hitting all of them in the same turn is not possible most of the time.
Nevertheless to average it, you will encounter 2 or more unit armies far more than 1 unit armies in which the warrior type hero has the advantage.

What about initiative stat? The WTHs start with more initiative than the CTHs because of this WTHs usually contest the first strike opportunity in a round. CTHs only attack before the healers, disablers or 2 unit size heavy hitters (with very few exceptions).
STHs have the highest init but the Clans STH even having better init than their other heroes, has less init than the other faction STHs
So, for me, the init stats are not a major factor for choosing a main.

The Undead
In the Undead saga, you will need to fight other undead parties, which can have death and weapon damage immune units in the same party. This is not as bad as it seems at first glance. You can have both death and weapon damage appliers with your hero and try to overwhelm death immunes with weapon damage and vice versa or you can use a Lich Queen hero as its main damage type is fire and you do not even need to main one and can just level up one in a mission to overcome this kind of encounters. Also the Undead has water and earth damage spells.
Advangates of each Hero Type
-Death Knight: only flying hero for undead, damage type is “weapon” so can damage almost all but some weapon damage immune units.
-Nosferat: unique scout hero, can gain a portion of its applied damage as life, as all scouts fast, If you can get some damage reduction banners of artifacts can tank in the frontline (I finished the saga with this). Damage type is “death” which is almost all the advanced undead units have immunity to, when you have a lot of undead units around to fight either utilize other damage type units or heroes, combined with “non death” damage spells. Has better damage potential than a Death Knight.
-Lich Queen: only hero among the Undead units and heroes combined that has fire damage as its damage type, great damage potential but slow on the map but gives the best option to deal with the other undead packs. Not a big deal but this can use staves and scrolls from the start, saves you from the necessity of recruiting another CTH.

My suggestion: Nosferat or Lich Queen are great with Death Knight is decent. All heroes can be main.

Clans
-King’s Guard: highest health pool of all heroes in the game with high melee damage but with low initiative and low mobility due to no flying. Damage type is weapon damage.
-Engineer: has the highest init among Clans heroes but lower init than the other factions STHs. Being the fastest hero for its faction, due to Clans overall slowness can be a good choice for maining but has the lowest damage potential. Damage type is weapon damage.
-Loremaster: Slow but has large health pool for a caster and great damage potential but has the lowest init among all the CTHs (I finished my saga with this). Its damage type being earth damage provides leverage against weapon damage immunity encounters. Not a big deal but this can use staves and scrolls from the start, saves you from the necessity of recruiting another CTH.

My suggestion: Engineer and Loremaster are good with King’s Guard following just behind.

Empire
-Pegasus Knight: only flying hero for the Empire. Since the Empire lacks high health units for frontline tanking duty can be used for that purpose. Damage type is weapon damage. (I finished my saga with this)
-Archmage: best damage potential for the Empire with having air damage type. Lowest mobility among its faction’s heroes. Not a big deal but this can use staves and scrolls from the start, saves you from the necessity of recruiting another CTH.
-Ranger: most generic STH in the game. Fast but walking unit with high init and lower possible damage than the other types. Damage type is weapon damage.

My suggestion: Archmage is the best with Pegasus Knight is good and Ranger is decent.

Legion
-Duke: generic flying WTH for its faction. Damage type is weapon damage.
-Counselor: only fyling STH of all the factions. Greatest possible mobility in the maps with high init and lower possible damage than the other types. Damage type is weapon damage.
-Arch-Devil: only flying CTH of all the factions. Great mobility with highest possible damage for its race. Not a big deal but this can use staves and scrolls from the start, saves you from the necessity of recruiting another CTH. Damage type is fire damage.

My suggestion: Arch-Devil is the best hero in the game, Counselor is great with that mobility and Duke is decent.

I only provide info about the equippable items since the rest are not competing for the equipment slots.

1) Banners- Best items for your main heroes due to providing party wide bonuses. Best banners provide 20% bonus for all your party which starts appearing in the second or third missions of the sagas. Banner bonuses are stackable. You can carry 2 of the same banner on the same hero and get all the bonuses.
Example:
Generic Empire faction hero party: 6 small units, 6*20%=120% overall stat increase. *2 for 2 banners 240% overall stat increase.
Generic Legion faction hero party: 4 units with 2 being big and 2 being small, 4*20%=80% overall stat increase. *2 for 2 banners 160% overall stat increase.
Best banners for me are:
The protection banners: best ones provide 20 armor. Extremely useful for empire but also great for the other factions too.
The damage banners: best ones provide 20% increased damage.

2) Artifacts- Second best item class. You can get the 50% bonus ones in the last missions of the sagas. Only effects your equipping hero itself.
The best ones you will get are 50% bonus damage and protection. No 2 of these are present in a map. But you can find multiple copies of the same lesser artifacts in a given map.

3) Tomes- There are 2 major type of the tomes.
-Elemental Ward Tomes: Wards are protection bonuses which negates all the incoming damage for the first attack that hits a warded unit in a combat encounter for the given elemental type of that ward.
Example: Fire Tome provides fire wards for all the units in your party. Combat starts and there is a fire damage dealing enemy unit and it can attack and hit one unit per turn. It attacks one of your units and hits that unit. That unit of yours takes no damage but if that enemy unit attacks and hits again that same unit, attacked unit starts taking fire damage. If the fire damage attacker misses its attacks, your fire ward is preserved until the first landing attack. A unit that hits multiple enemies can disable wards of multiple units in a turn etc.
I have never seen a life or death ward providing tome.

-Movement Tomes: 2 tomes, provide a bonus that overcomes the movement penalty caused by moving in forests or water bodies, each.

Tomes can be situationally good. A Clan hero wielding 2 movement tomes, can overcome their racial mobility handicaps to a great degree but tomes contest the same equipment slots for the banners and artifacts which are far great than the tomes. Elemental tomes are pretty situational, a fire ward tome does almost nothing against Clans or Undead etc.
In general I do not suggest picking “tome carrying” ability for your main heroes and even for your main support heroes.
I almost always take “banner bearing” for my main heroes with either “artifact wielding” or again “banner bearing” for my secondary and tertiary heroes.