On the street there are no fair fights, and you never know if the guy you're fighting has a knife/gun/years in an mma gym.
But as to the original question, I'd say kickboxing/boxing. I do not agree on any level with someone who even suggests its okay or reasonable to go to the ground in a street fight. Being on the ground in a street fight is death, terrible position to be in and I personally would never risk it.
I completely agree going to the ground is the worst thing to do in a street encounter. But I don't agree that kickboxing is the best option. I think a skilled kickboxer can rip a street brawler to shreds, but an unskilled kickboxer has a legitimate shot at getting KOed in an extended encounter. And there is no such thing as a guaranteed flash KO to end the encounter quickly.
With a standing grapplin art(throwing arts like Judo, Japanese Jiu-jutsu, or Sambo) you can get your opponent on the ground with total ease and then either run, like a normal person, or "stomp them" like a loon!
Of course this is all assuming the opponent doesn't have a gun or knife. If he does have a gun, give him what he wants and hope he doesn't kill you. If he's got a knife, hope your adrenaline inspired running the F*** away is faster than his crazed knife-wielder running!