Swadians are good if you time it right. If you join them just after they lost a pitched battle, you can free a bunch of prisoners and have a good army quickly.
Otherwise Khergits are better. Lancers are quick to train and they pack a punch much above their weight. They get horses quickly too, and have good riding skill, so your overland speed is going to be very good. Contrary to a popular belief, they don't suck in sieges. If you have companions with crossbows for support, you can just concentrate on Lancers and start taking stuff left and right.
Khergits are probably the best. If I joined Khergits in my current playthrough I guess that by this time we would have wiped out Sarranids and I would be ready to rebel.