Pokemasters.com (better when it had forums so they could help you with your team)
Bulbagarden.com (closed)
And then there was this one college site with the science behind Pokémon. Lots of mathematical equations which really helped in creating the perfect team. I forget the url though.
There's also MewHeart.com which is good for Pokémon icons but not for actual information. Then of course there's the official japanese site:
http://www.pokemon2002.jp