Any martial art can benefit you in what you want, whether it's self defense, fitness or discipline. But it all boils down to how good your instructor is of a teacher and how much effort YOU as a student puts into it.
Now, though some schools can be good, it is true that some can learn one aspect faster than another.
So for strict Self Defense you have:
Krav Maga: Israeli Street Defense, usually used in the military.
Eskrima: Filipino Martial Art that is trained with knifes and sticks and open hand combat.
Wing Chun: The Chinese art of trapping (I practice this) Great self defense if you find the right school, Luckily I found one of the 2 best kwoons in cali. Made by a woman.
There are also some places that teach plain old CQC (Close Quarters Combat) In general. They take anything that's useful and discard the useless.
Why did I not mention arts like Karate, Tae Kwan Do, etc.? Because though those give you the idea of self defense, they take longer to get good at, because you are being built physically, mentally, and spiritually to. The only traditional art(s) i listed there are Wing Chun and Eskrima.
Most importantly, find a good school, good teacher, and avoid contracts!
Oh, and Most of these can give you a great fitness workout!
Muay Thai/Kick Boxing is great fitness, but it only teaches one aspect of fighting.
BJJ is good to but it's the same as listed with Muay Thai, that's why people mix them up.
Another great pure fitness is Capoeria, it is a break dancing art but it very rare to find.
But in reality, most if not all (except sumo) martial arts were made for smaller people to take down bigger people! So you have more to choose from but most importantly, find a good instructor, good price, and spill your heart out in that school!
If you live in Southern Cali, I can possibly help you. Good luck with your journey!