As already mentioned, SPD if you spend any amount of time off the bike.
The limiting factor (for some) of all clipless systems but one is the amount of "float" (freedom of lateral movement), though most systems these days give enough for most people, from a biomechanical point of view.
Even given a small amount of float, the systems will always be guiding the foot back, due to spring tension, and this can present problems -- usually in the knees.
The one pedal/cleat system that avoids the issue altogether is made by Speedplay.
The Speedplay system moves the spring mechanism into the cleat and essentially allows unlimited float, without the tendency to re-centre.
The feeling for some is like skating on ice, but this soon feels normal. In fact, it feels natural because, especially out of the saddle, you are able to sway the bike without it yanking your feet around unnaturally.
I felt an immediate increase in power because of this freedom. And less knee problems because now my feet have re-assumed their natural (unnatural?
) gait.
What's more, Speedplay have one of the largest platforms of any clipless system, due not to the little "lollipop" pedal, but the big cleat area.
The downside is that you must avoid clogging the cleat with mud and/or gravel. "Cafe caps" are available and I just pop those on when I'm off the bike.