I've only tried a few different nations and only in singleplayer so far, but my favorite thus far has been MA Ulm. In my current SP game (my second game as Ulm, still learning to play them - the first was against only one opponent, this is on a 14-16 player map (apparently I only had 5 AIs on it)), I militarily crushed everyone I ran into except for Ermor and Mictlan (Edit: Eriu still existed too). Mictlan lives only because I stopped trying to kill them to focus all of my attention on Ermor. Apparently, the Ermor AI is smart enough nowadays to cast Burden of Time, then recast it again with even more gems if you manage to dispel it once. I dispelled it once, but was unable to dispel it after they re-cast it, despite three attempts with increasing amounts of gems each time. They're dead now, but only because my dominion overpowered them from the other side of the map.
6-12 months before Ermor ceased to be, I blitzed an army straight through Ermor's territory all the way to the castle their God was hiding in, only to have the entire army drop dead of old age within a month of reaching the castle - Although I had provided them with bags of supplies, I didn't have (at the time) any way to protect them from the aging effect. By the time Ermor ceased to be, I had discovered elixers of life and made several for my commanders, and had started pushing into Ermorian territory all along our border. Then they just went poof. I was a little disappointed since beating them didn't actually require doing anything except waiting for my dominion to overpower theirs.
Edit: Mictlan and Eriu just ceased to exist as well. I never even met Eriu. Looking at the scoregraphs, it's pretty obvious why... My dominion dwarfs everyone else's.