Warrior, because it's just more fun for me to be aggressive instead of defensive or magical. In terms of gameplay? All 3 are great. The Warrior is great in clearing the first swarms of enemies, the Mage's can easily pick people off, deal damage at range, and cause elemental effects to enemies that disadvantage them, and the Cleric's can just heal everyone and generally save people.
The lack of one of the 3 will mean you will lose. You will be severely lacking on one aspect of the 3 main combat mechanics, and you WILL lose. Unless you're fishing.
In any case, the best Combo, for me is to have, on a bridge that everyone needs to get past, one Cleric put up a Troll Wall to block all the enemies from getting through, and a Warrior using Charged Attack. All the enemies will be forced to run into the Troll Wall, and the moment it breaks the Warrior can release it, which will 1 hit just about EVERYONE in the area. If their entire team was there? The Warrior kills their entire team in one hit.
Tadah.
I got, like, a 7 kill streak from that move once. Thanks to whichever cleric that was, you made me MVP.