As the martial arts were turned into sports, their combat part (the "jutsu") was reduced to a minimum. I think that even if they dont make a good sport, it is better to learn the more combat-oriented martial arts, because they teach how to defend yourself better out in the street, where the situation is way more chaotical than inside the dojo. At least in the japanese martial arts (I am not familiar with the others, so I cant say anything abut them), the combat sistems are all related to each other so you can easily apply what you learn in a combat oriented m.a. while learning a sport one, and vice versa. So, if you practice aikido, you should also learn some aikijujutsu (the martial art from where aikido was originated) to learn the abilities aikido lacks.
I say Ninjutsu (that uses the taijutsu) is the best, and if you know ninjutsu, any sport marcial art (at least the japanese ones) shouldnt be hard to learn.
You shouldn stick to just one martial art, because none of them have everything.
PS: Today my english is terrible. Please excuse me for the grammar errors.