France I'd say was toughest, because she has some decent but not great lands - good enough to be desired but not good enough to be an immediate powerhouse.

Portugal, I'd say, was alright - with a bit of luck she could survive and do very well. Ally with Spain immediately, attack the moors to take Corduba (such a lovely province!), build up defensively, take on the Spanish, and they're sitting pretty in that peninsula.
If you leave Spain to itself, just remember: in MTW that other little Spanish faction always did alright for itself - not that I can remember the name!

All the others, I think, can just about survive. France I see as most likely to go out of the game.

Although I suppose the Holy Roman Empire is in a very similar position to France.

Edit: obviously it depends where the player starts. I guess I'm thinking in a game with no player influence. Can't of course say that for sure without seeing the campaign AI.