Do you have a favorite rival faction?

I like the Mages (Pariden).  Their hero (Xii) is one of the best heroes in the game, probably a tad OP to be honest compared to the rest.  That's really the one reason to like them.

fire elemental rock

The Ice Lords! I love that they hate the dwarves and their look is very unique. I feel like their kind haven't been done before, which makes them a bit special, too. 

Ice Lords, definitely. I have a particular affection for characters that are brutish/monstrous looking, but have an intelligent or mystical side. Ice Lords just have that primitive, but honorable and intelligent, brute kind of feel that I just dig.

Notable second goes to the Keepers of the Flame. I just love their dialogue, they definitely give off the vibe of a faction that knows a lot more than they are willing (or able?) to convey.

I like the Imperium the most. Their quests were interesting, I liked the idea of infighting and several 'factions' and fragile alliances trying to stay afloat. I also liked the visual style of their units and the hero a lot, even though the hero mechanics are hard to make good use of in larger formtions.