Whether you're talking fitness or self defense, I'd honestly suggest Muay Thai and Boxing. From a fitness standpoint you'll get great conditioning and develop lean muscle, and also work your balance and coordination.
From a self defense standpoint, both will teach you the importance of distance and awareness, hopefully avoiding the need to ever get into a grappling situation.
In addition, if you're limber and reasonably athletic, Muay Thai will teach you how to use your legs in a devastating manner. I've seen guys who are 5' 5" deliver knock down teeps (push kicks) to the chests and jaws of guys over six feet tall before they ever got close enough to grapple.