Speedplay Light Action's or Zero's.
I started with the Speedplay x-series - I found they would foul up with dirt and be harder to clip into with the slightly provocation. While I liked them otherwise, my Speedplay Light Actions don't do that, so I would suggest staying away from the x-series and going with the Light Action's or the Zero's (currently have the Light Actions on my bike, was wondering if I should have gone with the Zero's so I could adjust their float).
Though on a slightly different note, I recently got a pair of Time ATAC-compatible pedals that I really like. They're "mountain bike" pedals which have a smaller cleat, but that means that they can also have tread on the shoe on the sides of the cleat, so they're easier to walk around in. With older more flexible shoes that seemed to mean hotspots sometimes, but with modern shoes with super-stiff soles I don't think it's really a problem anymore. I've thinking about (though I don't know if I'll do it) switching my road bike to the ATAC's just so I have one less pair of shoes and cleats to worry about.