Here are some ideas on what could be used during gameplay.

PvE: based on your current experience/gear depends on gear and solo/group setups
best Tank - knight/warrior
best Heal - druid/x (druid has more support than priest) /rogue is current meta for crit-debuff
best MDD ranged - mage/warrior (can´t compete with current meta combos)
best MDD melee - warrior/mage (really good in long fights)
best PDD ranged - scout/x (x = warden, rogue or warrior)
best PDD melee - Rogue/Mage, Warden/Warrior, Warrior/Scout, Warrior/Warden, Champion/Rogue
best Support - warlock/mage, mage/warlock, warden/scout


PvP: also depends on solo/group play and gear dependent, but mostly what's most strong now

warlock/champ - very fast, high damage, good sustain
knight/mage - magical immunity, fast, good sustain
rogue/priest - immunity, invisible
warden/druid - good sustain, good damage
priest/knight - good heal, good sustain