Psy Fighter is top: they have the greatest area attacks dealing maximum damage for the enemy.
Assassin: if trained up, great against potent enemies towards the end of the game, also very mobile, so can be used to mop up loose ends in battle.
Spearman: choosing between non-Psy units, he is the best to do the tank job, higher health, greater damage and has a semi-ranged attack to compliment the arsenal.
Shadow Guardian is neat, but comes late in the game, by which time I already have a full, well trained team, so he's releatively weak in comparison, plus the Psy Fighter will have the same attacks, and they're unlocked already.
The healers are useful, but they're support units, really, and their role is secondary in battle. Shadowlings, Spriggats, Spirits- all great flying units, and can do fair damage if need be, but they're mostly used, by me, for "massing" effects, tackling the enemy with numbers.