I'll give you the advice my daughter typically gives me. The best team for any Pokemon game is the team you want to play. I'll add that I don't typically go for mono-type teams, particularly in earlier Pokemon games. It's easier to swing monotype teams in the later games since they've added so many dual type Pokemon as they went along.
I will say this, there's a lot of water types in Gen 3, so you're probably going to want a grass type.