On the defensive side, Deoxys -D is definitely the best. However on the offensive side, Deoxys-A is the best.
Deoxys-S is one of the best suicide leads ever, with Focus Sash and Spikes, it's almost guaranteed a set up. It's no slouch offensively either.
In my opinion though, I think Deoxys-A is the best, becuase of it's extremely high speed, attack and amazing coverage options. This comes with a fragile and useless defense as well, but Psychic type is anyways not really suited for defensive roles.
All forms of Deoxya have their niche, so it's up to what you want.
Hope I helped!