Depends. I always choose Swadia and Tevarin. Swadia is usually weak so aren't as strong fighting back as the other faction. However they are weak for a reason, exposed to all the other factions and are in constant war.
Other factions are stronger and it depends on your play style and what troops you like. I.e Remember massive morale penalty for Swadian troops if you attack Swadia.
Khergit: Hard in a pitched battle but much easier to defend against in siege.
Nords: Hard in siege. Solid foot soldiers. But if you have mobile archery army you'll own them in pitched. Just don't let them siege.
Rhodoks: Always own me. Their crossbows are just horrible in any battle and they defend their territory well.
Vaegirs: Never actually taken them on. But once you close the gap they are weak. So sallying forth in a siege I'm guessing beats them.
Saranid: Territory filled with those damn Black Horsemen
! I always avoid this area because you have to deal with these bandits and armies. On the bright side they kinda work as a patrol for other factions armies but eat your trade as a cost.
Edit: Oh and your villages are going to be in constant raid and ruin. Deal with it
. Nothing you can do.