Depends on your build somewhat.
- Ninja's Massive Strike is the strongest first turn nuke for non-DA builds, capable of killing glass cannons in a single turn. It also has a three-hit stun, which is much better against monsters with immobility res.
- Necromancer is a good defensive class for long quests - it has mp/hp heals, a long-lasting shield and a multi with no cooldown. Not to emention its passive, which stacks a reasonably powerful DoT the longer the fight lasts.
- Paladin is a fairly good defensive class against solo monsters, particularly undead ones. It has loopable boost and bonus nerfs, a light res nerf (also loopable), and a somewhat weak nuke that can, however, be used every other turn. The passive is a little HoT, which stacks with each turn.
- Technomancer is a good choice against mobs that use shields, and low-damage high-hp ones. It has a bonus and crit booster, a highly accurate multi and a 3-hit stun. Additionally, its passive allows you to deal increased damage the lower your mp is, to a max of 200% extra damage. That makes even regular attacks into nukes, and nukes into something quite op.
As for Caitiff, there's a few quirks you need to account for. First of all,, I suggest temporarily restatting to get 100+ End - you're unlikely to survive with a low amount hp. Secondly, use a class with multi-hit skills and nukes (to counter the shadow mechanic). I used Guardian, but any class with a low-cooldown multi-hit skill and powerful nukes works. Don't bother with DoTs, and make sure to bring lots of potions!