multiple opponents? I realized boxing and kickboxing and mauy thia focus on one opponent in a ring but karate focuses on self defense in their katas and the katas are hypothetical ways to defeat more than one opponentt.
Arts that teach you to defend yourself against multiple attackers are arts that do not require you to do too many holds thus leaving your hands free to block, parry and strike. Arts said to give you that freedom are: Kenpo Karate, Jeet Kun Do, and some forms of Hapkido.
The best way to win a fight with multiple opponents is to be able to run fast. There's no style that will make you capable of consistantly dealing with multiple opponents, short of Gun-fu, especially if they are skilled and/or armed.
To answer your question in the way you were likely hoping for though, if it came down to it, I'd say boxing would be the best, combined with Judo. In the unfortunate case you end up in a fight with multiple opponents, you're going to want to know the basics of all ranges of combat. Stand-up so that you're able to run when you get an opening. Clinch fighting to avoid getting tied up by one opponent (if, for example he does go and clinch with you) so that the others can surround you. Groundfighting in the situation you end up on the ground (maybe you get knocked down by a lucky punch, or you get tackled?) so you can get back up to your feet quickly. You'll want all your bases covered and boxing and judo would do this, while keeping it as simple as possible. When fighting multiple opponents your goal should be escape rather than beating them.
Biggest thing to remember: This is not Hollywood. Your opponents will likely not be tumbling over themselves from one attack, and they will not be coming at you one at a time. They will be trying to surround you, come at you from multiple angles, and will tire more slowly than you (afterall, in a fight with six opponents, you're having to expend six times as much energy fighting them off as they are!)