Kind of depends on what you want out of the martial art.
1.)You want to be able to actually use it in a street fight? [Muay Thai, Or a studio with mix martial arts studio that teaches all a combo of like boxing, muay thai, grappling, shoot fighting]
2.)Or do you wanna study it so you can put it next the many other cultural things you've learned in your life? Less about the fighting, more about the fun of studying something from another culture and time? [Various Chinese kung fu, aikido, capoiera, savate.....]
3.) Health reasons. Improve breathing exercises, coordination, energy, etc. [Tai Chi, Kung Fu]
4.)Or a bit of 1, 2 and 3? [then perhaps a kung fu or karate teacher that teaches both traditional and solid street application - hard to find though -]
Many martial artists bicker back and forth as to which is the ultimate martial art and which is the most effective. In my opinion, almost all of them are worthy of studying. It depends on the teacher and the student. I can't stress the importance of a good teacher enough. One that is grounded in realism, real world experience, wisdom, etc. Not some immature guy who is basically a child grown older. Y'know that guy with nunchuks in his highschool locker, but now he's 39 years old and runs a studio down the street from your Mom's nail saloon. With maybe exception to capoeira, tai bo, or any McDojo like places represented in "Napoleon Dynamite" as Rex Kwan Do, most martial arts are good. Again, the key is a good teacher.
Try to look for someone that comes from a valid lineage of other teachers. Don't be afraid to ask about the teacher when visited a place. One guy who studies boxing may make fun of one guy who studies Kung Fu. But you never know how that fight will turn out. Boxing is a great way to defend yourself. But what if they boxing teacher that guy learned from sucked? And what if the Kung Fu teacher the other guy learned from was really a good teacher? A man who is skeptical about the street application of Kung Fu may watch the fight and be in for a surprise.
Again, the teacher and the student are big factors. Martial art is third in line of importance. I mean yeah, there are martial arts that are built upon more realistic goals to fighting, but any line of good teachers and styles have upgraded and modernized their way of fighting to be 'street effective'. Like I said, just don't go to a McDojo and you should be okay. Interview the instructor. You should be able to get a vibe off of him. Even talk to some of the students too maybe.
p.s. another to consider is age. Is it something you want to use as young man? Or is something you want to still do even when you're 55? Muay Thai isn't the style for you if you still want to be busting opponents in your elder years. Even though its one of my favorites and I believe it to be in the top teir of effective styles, realistically speaking ...it is hard on the body.