First would be Jujitsu, wrestling, or Judo. ALL(unless the fighters previously said they won't be going to the ground) UFC fights go to the ground, and if you have a weak ground game, you're toast. Also ground martial arts like wrestling, in particular BJJ and Judo take YEARS to learn and master. It takes AT LEAST 7-10 years for your black belt in BJJ or Judo, unless your a natural at it, then maybe 5-6.
Next you need a good stand up game.
Muay Thai, Karate, Boxing are 3 popular stand up martial arts. There are more boxers in the UFC than Muay Thai, Karate fighters. Karate is ALSO another sport that takes YEARS to master, and if you can't find a good school, then you might as well do something else. Unless you're gonna be doing BJJ and Karate at the SAME TIME, don't even bother with Karate.
Muay Thai is another good art, but like I said, unless you train in Muay thai at the same time, don't do it because it also takes a long time to learn and master.
Boxing would be a good choice. If you make it to the UFC, you will have trainers to fill in your holes. Boxing is good because you learn how to avoid a punch and plus it's the best PURE(as in 100% focus) punching art. You just punch, no elbows, knees, just punches.
It's good you're not going to one of those "MMA" schools that do first 30 min grappling, next 30 min striking, and no stretching. They're just built by joes who watch UFC and think they can make big bucks off you cause of the sudden UFC surge. Go to individual gyms that focus on a specific art if you ever want to make it to the UFC.
Good luck, I hope I helped. YOU MUST START WITH YOUR GROUND GAME.
If you have a choice between ONE art right now, it HAS to be ground, and the preferred choice is BRAZILIAN JIU JITSU.