It all comes down to preference. The one thing that I really like is an open toe. I like to clearly see where the needle goes into the fabric. I don't like the Big Foot ones or anything with the clear plastic encircling the entire stitching area. I bought an $80 free motion conversion kit for my Janome where there are three different feet to try. And you know what? I found that my favorite FM foot is the basic darning foot one that came standard with the machine :) Figures.
Check out Leah Day's site on FMQ. She even gives tips to customizing a foot if you wind up with one you don't like. Good luck!