Majority of fights go to the ground. Brazilian Jiu Jitsu is the best method to learn to give you control when that happens. It trains you to be comfortable in any position on the ground. However, ask any decently trained fighter and they'll tell you that in a street fight, you dont want to go to the ground. In a bar or street, going to the ground leaves you terribly vulnerable if that guy has friends. Plus, other really random things can happen, blow out a knee dropping down on concrete. Getting spiked on your head while going for a lock. Shit happens, your best bet is to become proficient in BJJ in case you ever end up on the ground, but you should focus more on not being taken down. You're not gonna want to hear this, but your goal in a street fight isnt to win, it's to get away. There are too many uncontrollable factors, it's not like being in a ring. Dont take the chance, piss off the wrong guy, he pulls a weapon... suddenly you arent just looking at getting your ass kicked, but possibly killed.
Go take some BJJ classes though. It'll do wonders for your strength and cardio, plus it's a ton of fun. Aikido is a great style, but unless you are at the upper echelon, it is virtually useless. You have to spend years practicing before it becomes second nature and it can possibly be used for self-defense. Taekwondo is great for strength,speed and flexibility, but again is absolutely useless in a fight. Nobody kicks higher than the knees in an actual fight. Depending on the style, karate is decent but again, there are other styles that are far superior. If you want to be a great all around fighter, brush up on your BJJ and Muy Thai. Forget boxing, Muy Thai is where it's at in the MMA world. Hope this helps.
I'm a brown belt in Brazilian Jiu Jitsu. 3rd degree black belt in Taekwondo, and a 1st degree black belt in Kung Su Do. <-- Not bragging, just listing credentials and experience to lend credit to my opinion.