I've used Look, Shimano and speedplay.
Speedplay, light weight, dead simple to use if you get the Zero model its adjustable release angle and float, dual sided pedals means just foot on and click, downside is getting the mounting screw tension right for you can be a little long winded plus 4 hole fixing so have to use supplied adaptor, use coffee cup covers to walk in as metal base means you can slip easily when walking.
Shimano, neat, simple and light, 3 hole mounting cleats, pick colour of plate for float, cleat wears quite well when walking and as they have softer edges don't slip when walking.
Look, utilatarian, work well and simple, cleats do wear quickly so expect to change them at least once a year more if you ride loads.
Of the 3 speedplay are my fav due to dual sided and full adjustability, next up shimano cause they are easy to walk in and do their job when riding. Look's well if I have to, never been that impressed even with Keo's, maybe i'm just a snob