Wing Chun is my favourite one. It is very methodic and the movements are not instinctive, and as such, you actually need to train to forget old habits, which is one of the reasons it is so effective. It has rules at first but then discards them as you progress. It has what I consider the best sparring method (Chi Sao), and most importantly, a group of very defense-oriented rules. For example, the
center line, one of the main pillars about it. It allows you not to leave a delicate part of your body exposed, even in the event you miss a punch, and that allows you to throw a second punch right away, and not leave your opponent any time to think. We used to train blindfolded to develop the necessary reactions (in what it's called "sticky hands", because in Wing chun, if you manage to get each of your arms in contact with your opponent's arms, the ball is on your ground). There is nothing of that little hops you see in a fight, like two guys in guard, hopping in circles, waiting to make a move, in wing chun you go straight to your opponent. Wing chun is focused on close quarters. As my Sifu used to say, your main concern is getting into a comfortable position to fight him, which is elbow distance.
Krav Maga is very very effective and what most people would want to learn. It borrows from jui-jitsu, judo, boxing, among others. Real life is its main concern and the Israeli army is trained in it. Personally, I walked away from it because I think it is perhaps too risky in its aggression, the object that made me think so were the punches, since KM was influenced by boxing, some punches require you to turn your body, the result is stronger punches, capable of knocking an opponent down, but if you miss one, you don't end in a very convenient position; and besides, Wing Chun suited my personality better. Nevertheless, it is the one I would recommend.
I didn't personally train in Kali/Eskrima/Arnis, but I read about it and they are regarded as having some of the best knife techniques. You are probably familiar with the moves because Matt Damon trained in them for the Bourne movies.
Systema is a very tough one, you get punched for real, and teaches you to think out of the box, which is a MUST. But I'm not very keen on it because since you need to be relaxed at all times, you train being relaxed and your classmate does so, and while it is good in real life, it is not so in a class because you can get spoiled into thinking that your opponents is going to be as docile as a puppy, or in this case, as the person training with you, and then be surprised when you simply couldn't throw him to the ground because he is stronger.