Anything wizard is off the list, even troll wizard can (eventually) end up unstoppably powerful because wizard is just so good. Same goes (to a lesser degree) for any of the book caster classes.
Farmer is supposed to be a weak class, but it's actually not completely terrible.. polearms are very powerful due to the high defensive bonus, a tower shield and a spear can make you nearly unhittable if your polearm skill is good and farmers get a bonus to polearm skill training. They also get some good skills (herbalism, fletching, archery) and start with herb seeds.
Troll healer is actually really powerful - high base HP, built in troll regeneration, built in healing skill, plus healer (iirc) gets a bonus to healing.... you're very very hard to bring down.
I'd say you're probably right about troll elementalist though. They need to level up to get new spells and they aren't a book caster class so they don't get a bonus chance to find books or read them. You will constantly run out of castings of the spells you have, and won't realistically be able to level up enough to get the good high end spells.
Troll mindcrafter is a close second, they have to level up to get powers like elementalists (and read spellbooks even worse) but at lest they have unlimited castings once they get it so they are slightly ahead (imo).
After that, probably something like a (grey) elf thief, low hp = lots of dying, combined with (arguably) the worst class... yeah.