I can't really pick. Judging from pictures and whatnot, I would have to say Alton is the best, tied with port Aventura.
Alton just has everything. It has the scenery, the (beautiful) gardens, a lot of good looking roller coasters, and pretty good themeing. It's definately the park I would want to go to the most when I come to the UK in a few years.
Port Aventura has a few Dragon Kahn, which is probably the world's best sit down roller coaster (well it looks like it would be), and it has one of the world's best drop towers, that I'm absoloutely dying to get on.
So yah, those two are the two best in Europe in my opinion.