I can't help but say muay thai even though none are better. Its a personal opinion that you shouldn't study under what one person believes is most effective. You have to follow your own path. You need an origin, one art you have truly mastered before you should practice another, that way you can find what suits your own needs. Arts that are already mixed together just muddles the concept of each of the individual arts. Jeet Kune Do was Bruce Lee's. He was happy to teach people what he thought was most important, but I'm sure he wouldnt advise learning what a small group of people different than yourself say. Do Muay Thai first. Then expand and integrate what you learn next into the Muay Thai. Or whatever style you believe will work for you as your base.
MMA is not a martial art. It is the concept of mixing martial arts. That is usually Muay Thai and Brazilian Jujitsu. There is nothing better, as that term is highly subjective as to what you seek out of doing them. Find a good school.
As Man of Faith pointed out, MMA isn't a style, its a martial arts system of combining two or more martial arts. Muay Thai is often the preferred style of stand up chosen for MMA. So it's kind of hard to give you an answer, since they are almost one in the same. Only difference is that MMA is more well rounded.