MMA isn't fighting, it's a sport. The one interesting thing to come out of it from a martial arts perspective, was Gracie challenging the idea that grappling wasn't an effective strategy even against people with a lot of striking experience. But it has rules, it's one-on-one, no weapons ... it has very little to do with real-world fighting.
That said, someone that's been boxing for a few years is more likely to win against someone who has no form of fight training. But he might not win against someone who's been in regular fights the past few years.
People that actually fight, do so to win. That usually means two, three on one, bringing a weapon against someone that's unarmed etc. Being able to choke someone out doesn't help all that much if his buddy is going to knife you in the neck while you're doing it.
As for Aikido, I love Aikdo. And if trained for self-defence, it could probably work well enough - but that requires training in an environment where you have people around you to that know how to use the attacks your trying to defend against. I've rarely seen an Aikido dojo with people that are good at kicking, kneeing, punching and elbowing people. It's not even that often you'll see people with a decade of real judo practice under their belt. Without a few decades worth of such experience, it becomes impossible to truly practice Aikido techniques with an aim for self defence. If nothing else, there's a strong tradition for practising defence against multiple attackers in Aikido.
On the other hand, in terms of being fit, clear of mind, of freeing oneself from the idea that one has something prove, letting go of ones pride -- those are things that can certainly get one out of fights. And situational awareness and not being a moron is probably 99% of the key to successful self defence.
After all, if you know you're headed to a fight, you'd be better off going the other way, or pre-emptively calling the police, or getting some help - so that the fight can be avoided.
As for Karate - it can certainly be a good foundation for self defence. But there are numerous styles, and numerous ways of approaching Karate. If your focus in competition sports, it's unlikely you'll end up great at self defence.