Terran, protoss, or zerg. Probably protoss, but maybe terran. Actually zerg. Honestly tho it all depends on the opponent's race and the map, rush distance, whether or not you scout effectively... if you're facing a terran that walled off quickly terran might be best, as your marines could out range his... Why would you try to finish the game as fast as possible tho? That's not fun..
I find terran has the best rush with early (send one of the fstarting SCV's) proxy. Zerg rushes are expected and easily stopped by wall-in. Toss rushes (zeals) are the most powerful, but also can fail due to enemy kiting skillfully.
It is supposed to be zerg, but it isn't. cannon rushing > zergling rushing, and reaper rushing > zergling rushing. *grumble grumble* but if you are in gold or above, he could easily stop you and you would be falling behind in economy.
I vote this although my experience isn't the highest here. When I play against a zerg player I feel I need to start different and expect a few of them coming. If I dont, I lose in the first 5 minutes.
zlot rush is strong and hard to stop, if in good numbers but slow reaper rush is fast but weak marine rush is fast, lots of dps, but weak zergling rush really fast but weak TIP: a rush is pretty much an all out method to end the game right then, a fail rush could easily lead you to a loss, so you need to micro your rush a lot. like everyone said, it depends on what race you are playing against and what map.