You forgot a few, like the Nines, the Red Jacks, etc.
Also, Luigi's club?
Anyway...The Diablos, but only because it's easy to kill a lot of them at once with a shotgun and they have really cool cars. The Mafia is pretty cool too, but their car sucks
i would have to say the most deadly gang are the Cartels...only because they carry around the AK-47's. The coolest gang are the Mafia cause they look cool and they drive fast cars