Actually, you know what? I've got more to share than just a recommendation of one art. It's important to make a distinction, and it needs to be said among the peanut gallery parroting "BJJ!" every time this topic comes up.
I chose a striking art when I had to make a decision because I didn't like the idea of being stuck to one opponent – an open target for his friends to finish me off, if he has any nearby. My school teaches Arnis and Silat, but I'd learn to fucking box if that was the only thing available.
Rather than write a thesis over this, I'd have you watch this video. Look what happens after a brief exchange of words over a stupid basketball game. Black man throws a punch. Okay, fight's on. White guy apparently knows how to grapple. Good for him. Black guy apparently can't stay balanced, so he goes down with little assistance. White guy sets a lock in the heat of the moment…
…until the man's friends decide they don't like the white guy getting the upper hand. Several of them pull at the white guy's limbs to free their friend. When they pull the jiu jutsu lock away, the aggressor gets back to it and tries again, with no one holding him back. The guys who "broke up" the fight are fine with letting this happen; their friend "winning" is a more palatable outcome, until the more skilled one of the two initiates and successfully sets a lock again against the unskilled brawler.
Against all likelihood, the man in green finally gets into a position where he gives his assailant the chance to back down and walk away (which never happens) and AGAIN the others are trying to find a way to break them up just enough so the assailant can continue his assault. He's about to show mercy and they're still trying to devise a way to make him lose.
This shit happens all the fucking time when you go to the ground with other people present. You don't know what they're going to do.
At any point, any one of those spectators could have stomped or stabbed the guy trying to set in the lock, which illustrates a key failure of BJJ or any other ground-based art: many times, it fails to address multiple attackers.
Yes, by all means, learn BJJ. It is a wonderful art with many principles to teach the end user. But for the love of God, don't make that the only fucking thing you know. It is a tool in your toolbox, taking its place among all the other tools. Nothing more.