They all serve different purposes.
Bulbapedia is where you go when you want EVERYTHING and ANYTHING. Movesets, learnsets, types, locations, lore, characters, every game, every episode, every manga, ever card, you fucking name it it's in here. Gotta find it, though. It's where I always go, because I know that no matter what it is I want to know, if it's anywhere, it's here.
I haven't used PokemonDB but it looks like it's a lightning fast, no-bullshit stop for stats, moves, locations, and almost nothing else to get in the way. Glad to have it for those who only want that info, which is probably most people most of the time.
Serebii is one guy trying to be Bulbapedia all on his own. His autism is commendable, but he's no match for the autism of a community of wiki slaves, and it shows. His website is ugly as hell, poorly laid out, full of errors, sloppily written, and generally incomplete. Unless you're using it only for the news (which being an uber-autist with great connections gives you an insane advantage) I genuinely don't know why people bother with it or any other half-baked fan site or wiki in current year.
