In no particular order:
1. Sarranids, for the reasons KiriKaneko so ably lists.
2. Khergits, because the AI can't play them worth a damn. Their strengths are mobility and archers, but the AI never brings enough of either to be effective. Plus, their armor sucks (especially their helmets), and they fold like cheap lawn chairs in a defensive siege.
3. Swadians because, as noted, they usually get ganged up on and weakened enough for you to deliver a really easy death-blow. Be prepared for everyone and his limping uncle to come after Dhirim constantly, though!
4. Vaegirs, for pretty much the same reason as the Khergits and Sarranids: the AI doesn't usually bring enough heavy-hitters to the table. Their 2-handed infantry carries good weapons that pack a big wallop but lacking shields, they're very vulnerable to missile troops. When their infantry spawns with shields, they're worse: bad shields and meh 1-handers. Their archers rock, but suck in melee and thus are vulnerable to infantry and cavalry, all the more so since it's so easy to kill off the infantry that's supposed to be protecting them. Their cavalry is OK, but aren't often on the field in big enough numbers to turn the tide (same goes for their archers). Most of their armor isn't all that great, either.
Really, the AI mostly sucks at maximizing the strengths and weaknesses of all the various factions, though the Nords and Rhodoks are, as always, the toughest.