Biological, starightforward and deadly. It's impossible to beat a good Biological in the PvP arena (highest speed-> crystalize enemy sonny, win).
After Bio I like Hydraulic, namely Hot Hydro, as every attack from Hot Hydro hits very very hard.
I like Psycho least, electric psycho is good since shock therapy is such a powerful skill and is accessed so early, but it's power fizzles out quickly.