I'm looking at this from a self-defense point of view....not mma as a sport, for the record
1) Karate (only if trained properly and not "sport karate") -- a simple, effect art that has the basics, the building blocks, of many other specialized arts, its a good entry point for martial artists, and against your average joe gives you most of the tools you need to defend yourself
2) BJJ - Especially given the prevalence of ground fighting in todays popular culture and, there is a good chance you'll end up on the ground and you need to destroy some limb or choke the person and get back on your feet and get out of there
3) Western Boxing - improved punching/striking over Karate in general. A simple jab, cross, hook, or uppercut is often all you need to defend yourself "on the street"
4) Muay Thai - improved elbows/knees over karate striking (although inferior punching and head movement to boxing) and its hard to argue against a hard leg kick to take either injure the limb or take the persons legs out from underneath them
5) Jeet Kune Do (Concepts branch) - kind of a catch all for anything that works....plus introduces new techniques like wing chun hand trappling that the above arts don't really utilize and nasty eye gouging