Tank is a very...wide word, in the world of istaria. :P
Cleric, while nice and the only class that can wear platemail and also cast the "prestige" augmentation/life spells (minor through to major health and the enhance stats), yes, is a very weak class and has awful abilties since it's kinda...meant to be a base class - supposed to be leveled so to make other classes better, that is.
Crossbowman, Chaos Warrior, Knight of Creation, Reaver, Paladin, Cleric, Battlemage and Warrior can wear platemail, the best of the armours.
Amongst thouse, cleric and warrior are base schools so aren't really ideal. Crossbowman is...nearly pointless too and outranked by really, all of the listed classes (heck, i think even cleric would be stronger...lol).
I don't have much experience with the other classes but i've hunted with all except knight of creation for epic hunts, and they're all pretty good.
I'd personally go with battlemage, but then, i'm a user of arcane classes so i'm biased towards favouring the only one that can cast all of the arcane spells too
Battlemage gives mages a sword and platemail, essentially, so you have access to all the arcane spells (flame, ice, energy and summoning spells!) and the swords and armour and melee abilities of a warrior!
Though, reaver is my favourite actual 100% melee class, since it combines the abilties of a spiritist with melee. So you get a bunch of health saps with it.
(well..more like 70% melee since it gets access to all of the spirit spells...but still!)
I'm pretty sure paladin has the most tanky abilities in terms of defenses and such? It's the only prestige class that wears plate that can heal others (via school gained abilties and apart from revitalise) as far as i know.
Reaver would do better at healing self only since it has access to spells which sap health from the enemy and give it to you instead but can't heal others too well.