Muay Thai without question. Generally, going to the ground in a street fight is not a good idea unless you're in the middle of a large empty space and there's nobody around but the guy you're fighting. Getting kicked in the head by your opponent's friends isn't fun.

So if ground fighting styles are not the best option, BJJ is out. Of the standup arts, Muay Thai has always proven to be the most effective style of the styles that train with any real form of sparring. It could be argued that other styles are more effective because they teach eye gouging, groin strikes, etc. But most of these styles do not train with aggressive sparring and IMO, there's nothing that prepares you for a fight like putting on a pair of gloves/pads and having someone try to knock the snot out of you. Not only that, but if you gouge out somebody's eyes, you're in trouble unless they're MUCH larger than you or they have a weapon.