The rest of this guide is spoilers. Some of the early sections are minor spoilers about hidden features and mechanics in the game. The last section will include full details of the best builds I’ve found. I had a lot of fun discovering things in this game, so I don’t want to take that from anyone without warning.

While pushing as far as I could in endless, I noticed progress was not matching the DPS I was seeing in game. I would do some simple math (wall health / DPS * 3600 = hours to defeat) and it wasn’t coming out the same. That’s not too strange. The DPS in the top left has never agreed with the damage that pops up over the wall.
Eventually I noticed progress stopped entirely. This was definitely strange. No matter how long I waited, wall 178 of New Game would not fall. There was always 4.4 / 24.2 Oc HP remaining. After checking the save file, the wall damage never changed. This is the end of endless...

In later attempts, I found this number fluctuates a bit. It depends on how much damage the build is doing. It needs to be significant enough to affect the overall damage. It seems like the DPS has to be at least around .001% of the wall's HP (or 5 OoM). In other words, if a wall can't be cleared in 28 hours, it's likely impossible to get past.

Edit: damage is not totaled up over a second and then dealt to the wall. Each instance of damage needs to be significant enough.

Shortly before reaching the final wall, I was working on a small Python script to show the actual damage being dealt every second. The game saves often, maybe about once a second, so this script checks that saved damage every 10 seconds and averages out the damage.
Python script: https://pastebin.com/v1q85Eqp
The verdict: True DPS is always lower than the top left DPS. The damage popup above the wall is more accurate, but lag can skew even that number by quite a lot. The lowest FPS builds I’ve tested do about 25% of the popup, and maybe 15% of the top left DPS. More often, laggy builds deal about 50% of the top left DPS.

It’s not all bad, though. The corruption orb is truly powerful in endless mode. The corruption orb has a hidden, fourth effect. Every level of corruption orb gives immediate souls, using the same formula as the quests before the corruption orb did. With an orb level 50+, over 8k souls is achievable. Most units cap out at 800-900 due to mana cost, but summoned units and soul-expensive units greatly benefit from this. 2000 souls to 8000 souls is more than 4x damage, as we will see in the next section.

Part of what can make a build strong is how much it can get out of additional souls. A pure skeletons build has only one use for souls.
DPS = soulCount * skelDPS
After 1000 souls, adding another skeleton only adds .1% damage. If we add overseers
DPS = (soulCount/2) * (soulCount/2 * overseer DPS)
Additional souls’ effect is squared. Adding a soul after 1000 here adds .2%. Twice the impact.
Another example of a soul having more value is builds where adding a unit increases DPS and damage multi, like archdevils (assuming you have a sacrifice-based damage multiplier).
When allocating souls, the simplest way to optimize damage is to split them equal to the ratio of multiplying terms a unit represents.
In the case of an archdevil build, angels and archdevils both multiply damage (with forbidden contract). With 1000 souls and assuming A is the base damage of an Archdevil’s huggers
All ArchDevil: 66A
Equal count: 16A*15 = 240A
50/50 souls allocated: 33A*10 = 330A
But this doesn’t account for Damage multi. Assuming we’re only using sacrifices for damage multi, and D is the total damage multi provided by 1 archdevil:
All ArchDevil: 66A*66D = 4356AD
Equal count: 16A*15*16D = 3840AD (less than pure archdevils!)
50/50 souls allocated: 33A*10*33D = 10890AD
66/33 soul allocation: 43A * 7 * 43D = 12943AD
The best soul split gives 2/3 souls to archdevils because they add damage to 2 factors.

For the sake of argument, we can quickly rule out some of the damage multipliers that are just outpaced in endless

• Incinerate - 18x encantation effectiveness means 5ish damage mult per skill point. Not good enough.
  
• Mana Amp - Maybe if there was a synergistic skill that improved the base.
  
• Knights - could be worth exploring if it was possible to put all souls into knights, but it’s too mana expensive to afford more than about 900 of anything. Even then, knight’s damage just isn’t good enough and uptime of the damage mult doesn’t work for any other damage sources.
  
• Solitude - This assumes the damage source is an invocation, but for those builds captains/generals will always be better
  
• Strength in numbers - Even if it were 10 mult per 1 unit, other skills will outpace this.
  
• Bards/Jester - The simplest comparison here is captains/generals, which have more skill point investment available and both units can use an unlimited number of souls, where bards are capped to 50.

This leaves the viable damage multipliers

• Bone tsunami - doesn’t fit all builds, but good point value if you already have skeletons.
  
• Captains/Generals - The largest feasible damage multiplier. Sacrifice skills can maybe beat it, but not without dropping to single digit frames and waiting a few hours for fracture stacks. Even 60/60 skill point investment can remain competitive with sacrifice multipliers.
  
• Hell Gate: Allows for higher skill investment than volcano and fracture and doesn’t require sorcerers. The downside is it has the lowest total damage multi among sacrifice options. Also the lag ramps up around 60k-90k stacks.
  
• Volcano: Requires either active play or creative solutions. Caps out at 195k damage mult (488k after mult-mult) at 30k sacrifices/min. Causes some lag, but less than it used to. To run volcanoes passively, click at about 2.5 times per second until max stacks then start a hold down volcano. As the volcanoes expire, they’ll be replaced at an even pace to avoid small rupture count volcanoes.
  
• Fracture: Likely the best damage multiplier. 1.19 damage multiplier per sacrifice with no cap and very little lag. Every 10k sacrificed/min is 4250 stacks, or 60k mult (149k after mult-mult).

Here’s a calculator I made to tinker with the sacrifice options:
https://docs.google.com/spreadsheets/d/14VjLr7Wxvgh3SDk5fuPjTPJ0qFa7hL2-KPU76XvzH7Y/edit?usp=sharing

• Insects: 14 per click * 25 clicks/second * 60 seconds = 21k sac/min
  
• Chain Lightning: 17 casts/click * 5 per sac/cast * 25 clicks/sec * 60 secs = 128k sac/min ( laggy)
  
• Blood golem: Infinite. 900 recruiters at 60 skill points = 113k sac/min (laggy)
  
• Archdevils: 170/Archdevil. 250 archdevils * 170 huggers/min = 43k sac/min

Endless mode exists in all New Game modes. Lower NG+s will have lower HP for a corresponding wall. Every following New Game+ is about 5 levels harder at a given level. This means New Game would have about 60 more skill points at the same wall HP. The downside of New Game is the corruption orb only gives the initial 666 souls. New Game soul count caps out at around 2500 souls, where NG+Final reaches 8500 souls. In my experience, New Game +1 offers the best of both worlds, which extra skill points and extra souls.

After test many builds with late game resources (8.5k souls, 560 skill points), I believe there are 3 damage sources that stand above the rest, being that they are all 100 times better than the next best build. If anyone has found more success with one of these builds, I'd be happy to hear about it!

Likely the weakest of the top builds. I ruled out captains/generals because archdevils and angels have high soul costs and because Archdevils naturally synergize with sacrifice based damage multipliers.
If you don’t want to mess around with volcano setups, the fracture build can work, but the ramp-up time will make the first 10-30 walls slower. Later walls, fracture is better and less laggy. Both together might be the best, but that depends on the lag.

DPS: 20 Sx - 150 Sx
True DPS: 16 Sx - 90 Sx

Base build (224 skill points): 30 mastery 3, 60 archD, 60 hugger, 60 slice of hell, 10 recruiter, 1 angel, 1 sorc, 1 volcano, 1 boozooka

Pick one (or both later)
Volcano (61 points): 1 volcano, 60 reverse engineering
Fracture (60 points): 60 fracture

Max (204 points): 60 sorc club, 24 boozooka, 60 master 2, 60 slice of hell

Items: Forbidden Contract, Wade’s feather, mana fang, Truncheon or whatever (glove if you don’t alt-tab hold, waiting glass for orb levels, dragon scale maybe?)

Soul distribution:
If not using volcano: 66/33 allocation between archdevils and angels.
If using volcano: 66/33 allocation until 141 archdevils, enough to max out volcano. After volcano is maxed, 60/40 allocation, since archdevils are only increasing fractures, which is (about) half of the damage multi.

Dragon has a very strong base damage, but sacrifice options are limited since they need goblins and cultists for multipliers, leaving only insects. I consider captains/generals out of the question because dragons receive power from souls in multiple sources and splitting soul distribution between 2 more units would be worse than halving damage.

DPS: 100 Sx - 1 Sp
True DPS: 50 Sx - 400 Sx

Base build (284 pts): 30 mastery 3, 1 dragon, 5 mimic, 5 recruiter, 1 sorc, 1 boozooka, 1 insect, 60 goblin spices, 60 dragon cult, 60 bane dragon, 60 infestation

Pick one (or both later):
Volcano (61): 1 volcano, 60 reverse engineering
Fracture (60): 60 fracture

Max: (84) 60 sorc club, 24 boozooka

Items: Trucheon, goblin crown, dragon scale, wade’s feather / mana fang

Orb Feedback
Orb feedback is an odd skill. Orb Fixation’s click bonus is x*1.085^x, which makes orb fixation one of the only skills with an exponential growth rate. For this reason, it’s more worthwhile to invest in orb fixation than to max other skills.
My first run I chose New Game for the extra skill points. This meant sacrificing around 6k souls. I did another run with orb feedback in New Game +1 and found a new damage multi source in bone tsunami. New Game +1 ended up being about 10 times higher damage and less laggy.

DPS (NG+0): 100 Qn - 1.7 Sp
True DPS (NG+0): ? - 500 Sx

DPS (NG+1): 100 Qn - 22 Sp
True DPS (NG+1): 50 Qn - 10 Sp

NG +0 variant (sacrifice multi):
Base build (199 pts):30 mastery 3, 13 frac, 12 sorc club, 1 sorc, 1 blood golem, 1 blood comms, 1 noble, 1 boozooka, 22 gathering, 24 orb feedback, 24 wizard glass, 45 recruiter, 24 orb studies

NG +1 variant (Bone Tsunami):
Base build (151 pts):1 blood golem, 1 blood comms, 1 noble, 23 gathering, 25 orb feedback, 25 wizard glass, 25 recruiter, 25 orb studies, 25 bone tsunami

Orb fixation (450 pts): 30 mastery 3, 60 mastery 2, 120 mastery 1, 240 orb fixation

Maxing is not possible, just divide points equally after orb fixation is maxed.

Items: Truncheon, dragon scale, wade’s feather, mana fang

Souls:

• Summon lots of nobles
  
• Summon recruiters up to mana limit\
  
• Limit cultist summons to 0
  
• Wait for skeletons to max out
  
• Remove cultist summon limit and sell nobles
  
• Auto cast blood golems (lag will start)
  
• Sell skeletons until souls never hit 0 (recruiters have enough souls to max summon each tick).

Overall, I'm happy that builds can't push to infinity, and the highest wall achievable is limited to the build that can do the highest damage. This gives every build an end state instead of idling for eternity. My most recent endless push took only 3 or 4 days of idling before it maxed out.
If anyone has any suggestions for other strong builds, or has had better results with one of these builds, I'd love to hear about them!