I would say Muay Thai. If it gets into a clinch or within clinch range, as street fights very often do, a guy who is trained in Muay Thai is deadly, when you take into account the Thai clinch, where you can throw knees, elbows, etc..
In my opinion kickboxing is pretty uneffective in a street fight, at least the kicking side of it.. if you throw a spinning back kick in a street fight you're either gunna get caught in mid air and slammed on the floor or you're gunna land it and land on your feet off balance for a split second, where some other sucker could take advantage of it. On the other hand, Muay Thai is all about quick, effective & close range attacking, delivering as much damage as possible in the shortest time.
If it goes to the ground then yeah it would be advantageous to have some kind of experience in wrestling or BJJ, but only so you can use it to get back to your feet as quickly as possible, because you don't want someone stamping on your face as you're trying to triangle choke someone