Well, now that DP's out I guess using the DP one would be best, but if you want my honest opinion...
Veekun is a much nicer Pokedex than Serebii. ^_~Seriously, it lists all the attacks, abilities, and stats. It doesn't have different versions for different generations, which is much less confusing. There's a lot of extra stuff like breeding and capture rates, Pokedex entries and categories... Overall, it's a really nice Pokedex and I would highly reccomend it over Serebii. ^^