What's your budget? You''ll get a raft of people telling you how fantastic M520 Shimano are, but other pedals are available.
Here's my twopenn'orth
I started off with Crank Bros Eggbeater C's. They were on special offer at the time and I was told that they were easy to clip into, have great float and were very light. They look very good. I used them on all sorts of trails, and discovered they were great. Easy to clip into being four sided, and the natural float is superb. If you intend to ride smooth trails or lots of XC type stuff then I would have recommended them. I found their limits on rockier trails, as the design means that as they strike downwards onto rocks they open up and disengage. On some really hairy descents I lost a bit of confidence with them, and ended up getting some Shimano M545 which have a cage surround. These are much better in my opinion on rough trails, but are of course unnecessarily heavy for XC riding. They have a large alloy cage, and Shimano do other lighter versions.
I doubled the Crank Bros up on the commuter so I could wear the same shoes on either bike, and since taking them off the MTB I've gone onto PD A 520s, a single sided caged touring pedal on the commuter. I love the power transfer but miss the Eggbeater float......(or at least my right knee does!)
My son uses cheap old M505's or flatties, but then he's mad.
