I like krav,Im currently taking it,and I feel its very practical for a real world situation,its basically streetfighting,or thats atleast the best way I can describe it
I think that you will find that most MMA fighters have a bit of background in brazilian jiu-jitsu.
If your doing it to stay in shape, do what you enjoy, if doing it to fight, I think you need a bit of BJJ.
I've done a fair few, first you have to decide what you want to get out of it, then you need to look at the types that fit the criteria and try out a couple of clubs in that area.
For instance if you want a grappling art have a look at bjj or judo. Personally I would love to do bjj. I have mainly practices Kung Fu (Northern Mantis), Boxing, Muay Thai and a bit of ground fighting but I love boxing, my fitness was incredible when I was doing it and my punching power and foot work was excellent.
If you want more advice email me through my site at Combat Fitness and I should be able to help you.
I have to agree with Judo Tom there, he is absolutely right, you can have the best style for you but if it isn't taught right for you you'll never get anywhere. Instructor is the most important factor in the style to choose.
you should probubly find a school that offers training in both grappling and standup
for practicality
on the street you will likely want to stick to standup because if you are outnumbered grappling with one of them while the others beat onyou is only slightly more effective then curling up into a ball and waiting till they get tired of beating you
on the other hand if you only practice standup and get into it with a streetfighter that wants to roll (grapple) you will be in a bad way if you don't at least have the grappling ability to keep him off of you or work some escapes to get back to your feet
honestly Krav is more about marketing hype than substance , its really a poorly Slapped together "Best Of" system , and such things tend to suck IRL .
Think about this :
Everyone thinks that JKD is the greatest system ever - why is it that NONE of Bruce Lees students have ever produced a successful MMA fighter ? Not even ONE ? Because systems that try to be Highlight Reels tend to fail miserably against anyone with training in a REAL , TIME TESTED system .