I agree, trying to advance certain classes at certain ranks is a big pain in the butt. Then again, usually once you get a group it goes really quickly. Also, in most cases "big pain in the butt" doesn't mean its technically impossible, though in some it may.

My nominations for "most annoying to rank":

Pre-form shifter. Mitigating factors: you're probably human so you have zero xp penalty.

Pre-elemental conjurer. Mitigating factors: at least you can spam magic missile. Make mental notes of people who rudely refuse to group with you; if non-good, take great joy in gaunting them later.

Druid, all ranks. You're like a crappy healer. Mitigating factors: you have no xp penalty, although you're probably not human so there's likely some race-based penalty.

Shaman, all ranks. Even with sanc you're a mediocre tank and you don't do much damage unless you expend mana. Mitigating factors: you can summon. That makes ranking easier {and safer} in some situations. Also no xp penalty, except that you're probably not human.

Pre-enhanced-damage thief. You can't really tank and don't do much damage even with circle stab. Mitigating: no xp penalty, plus you can see hidden which is invaluable to groups that can't.

Necromancer, all ranks. You don't do alot of damage except in certain scenarios and are only a mediocre tank even w/ wraith form and vampiric. Mitigating: people want to be on your good side so you don't annihilate them later. Plus you can summon, and have no xp penalty (and are probably human). At low ranks you can spam chill touch for damage.