Seems to be quite a bit of talk about aikido so I thought I add a few comments for what it's worth.
Different styles of aikido have different focuses. "traditional", "tomiki" and "yoshinkan" are the big three in the UK. If it's traditional aikido, you'll focus on wrist locks and some pretty (but not overly effective) throws. If it's tomiki aikido (the British Aikido Association main style), there's a lot of randori (the grappling equivalent of sparring in Japanese martial arts like judo and traditional jiujitsu). Like judo (tomiki is a style of aikido based on judo competition), the randori is more sport oriented, rather than self defence. If it's it's yoshinkan, it's a bit more aggressive, technical, slower to learn and a lot like more the daito ryu jujitsu style aikido is based on.
To start with, most aikido will seem pretty artificial, as it takes a while to get the technique down. However, once the techniques are ingrained it's pretty effective as a martial art. I would say that this is probably true for most of the martial arts you've listed (I do/ have done judo, aikido, karate, Taekwondo and jujitsu and this applies to all these) - e.g if you did karate you'd probably be able to grasp the basics of a gyakazuki (back hand) punch after the first lesson, but it would take a few years of practicing it before it would really be effective in a real life situation. One final point on aikido - may not be relevant but it's one of the few martial arts I know of that can be effective as a smaller person against a big person (speaking from experience, when I was at university I once ended up in a situation where a guy that must have had at least 30kg lean mass on me started trying it on one evening and I instinctively did a nikyo wrist lock and he fell to the floor screaming)
As others have said, the best thing to do is to go along to a few different classes and see what you enjoy most.
That being said, I don't think karate or Taekwondo are what you're looking for. I've done a few different styles karate on and off since age 10 (I'm 27), and although it's interesting it's probably not the most practical striking art for self defence. Taekwondo is more of a sport than a martial art - the clubs I've trained at do a lot of pad work though if that's what you're looking for.
Hope this helps with you choices and good luck!