I like them all.
But, I had to vote for Donal Lugh.
For the Bannor, he's amazing. Kill a skeleton, run back to town, recruit a bazillion fodder units, rinse and repeat..
I leave barrows and graveyards up when playing Bannor, just to be assured I'll have a skeleton to kill in case I need a quick influx of units. As soon as one pops, I use his Recruit function in my biggest city then kill the skeleton to get the ability back. Rinse/repeat/win.
Recruiting a bunch of champions, horseman and assassins whenever they're needed is outstanding. In the late game, there are usually summoned skeletons and demons in any battle, so you're always assured of having Donal with his Recruit ability handy.
Note: A nod has to go out to Alcinus. The roleplaying opportunities he presents as you try to hunt the crazy, treasonous, so-and-so down are an excellent flavor addition.