Zealots aren't as much of a tarpit as they used to be in Mk2, due to the changes to the Monolith bearer. That one turn of immunity was far better at it than the turn of resurrection, IMO, since it gave you a chance to unjam them in the right lists/combinations and give them another chance to chuck bombs. With the resurrection, the part that didn't die will most likely still be engaged. Plus, you need to be really careful not to get the monolith killed along with the unit to even benefit from the resurrection, whereas in Mk2 the monolith almost always pulled its weight until it became expendable. Having said that, when played right they can still be an effective tarpit.
Cinerators are indeed too slow to be a proper tarpit. They're better suited as a defensive or 2nd wave unit that follows behind the tarpit.
With a natural high DEF, idrians and TFG are better suited to the role of tarpit, as both of them receive any defensive buff really well. Idrians are too expensive to be a dedicated tarpit, but their ranged attacks and offensive output makes them really versatile anyway.
TFG, being relatively cheap and pillow-fisted, are much better suited for the role of roadblock/tarpit, and their suite of abilities makes them really effective as well, giving them much more resilience than most other units on their points lvl.
Alternatively, and much depending on what you are facing, Exemplar Errants can also be an effective tarpit unit. Their self-sacrifice ability combined with the right support and defensive bufs (remember the Mk2 errant boat) is every bit as annoying and effective as it was in Mk2. However, fully decked out, they cost as much as the idrians, but don't have as much damage output as the idrians.
In short;
Cinerators: Poor defense, solid (albeit slow) offense. Best as 2nd wave unit.
Idrians: Decent defense, Good offense. Versatile unit that is best used as flank holders/skirmishing unit (hey, weren't they called skirmishers?
). THeir high speed and advance deployment helps a lot in all their roles.
Zealots: Poor defense, decent offense, one-time good recursion. BEst used as flank or short-term tarpit unit.
Errants: Solid Defense, mediocre offense. Best used with a warcaster that can support them in both offense and defense. Let them hold a zone/objective, then swap defensive to offensive buffs and catch your opponent off guard. Spd 5 is a bit low, but AD helps here.
Temple Flameguard: Solid defense, poor offense. In my opinion, the best defensive unit we have. 2" reach with their spears allows them to tie up entire armies, and their solid SPD lets them get there when they need to. Combined melee-attacks does allow them to fight back against tougher targets.
Additional options:
Daughters: Very solid speed and DEF score lets them take defensive buffs well. However, small unit size and low ARM makes them vulnerable, and their AD + high speeds runs them out of buff range easily.
Cleansers: poor defense, mediocre offense. These guys need ways to help them in any role, but even a good defensive buff will not lift them over mediocre defense. Their sheer volume of potential attacks is their saving grace as a second wave offensive unit.
Bastions: With 8 wounds and an ARM buff, supported by their seneschal, they were solid defensively. With 5 wounds they just die too easily to be defensive, and with SPD 4 and no native way to increase it, they're too slow to be offensive. In a vacuum they're not bad. Within the faction, they're one of our weakest units.
FLamebringers: Solid Defense, Solid Offense, incredible mobility. THis makes them excellent skirmishers. THeir greatest defense is the ability to avoid retaliation, and they're too expensive to 'sacrifice' as tarpit.
Vengers: Great offense, poor defense, good mobility. Not a tarpit. at all. They will die to retaliation. LEt them charge into something hard, then try to keep them safe for another charge. 3 charges would be living the dream.