Bulbapedia is, by far, the best website for any type of pokemon information. The only problem is that because of the release of Platinum, so many people have been using the website, so it crashes a lot. : / So I try to use the website early in the morning or late at night because less people are using it then.
I've tried serebii and pokemon marriland, and while they're not horrible, they aren't very good websites. Especially serebii, ugh. The colour scheme is enough to make my eyes bleed but the website isn't much better, either. >_>