Which kind of mountain bike pedal is most suitable for racing? It needs to be pretty light, but also durable to be able to resist a tough race.
I'm assuming we're taling clipless since we're talking racing. I've owned a number of SPD pedals and always found them lacking. I prefer Time Pedals and raced x-country on them for years. I own a pair of Alium's. When I bought them (5 years ago) no SPD kept as clean as they did. Speedplay and Crank Bros are also supposed to clear mud well but I've never tried them.
Ultimately this is a personal preference. See if you can get on a few different pedals at your LBS and read some reviews in magazines and on forums.
I have had SPDs and eggbeaters, and I love the Crank Bros eggbeater pedals. I've found them to be way better than SPDs. You can clip in on four sides rather than two, and in my experience, they're easier to clip out of in emergency situations than SPDs are.
Also, unlike SPDs, eggbeaters don't have flat surfaces, so you're less likely to slip off if you're trying to get back on after hike-a-bike sections of muddy trails.
The Eggbeaters as above pretty much win on weight, ground clearance and clogging up, but I have heard about the odd reliability issue, and they don't have adjustable tension
The Shimano SPDs have adjustable tension, even on the bottom end M520 pedal and are a bigger target to stamp a foot on if you have unclipped and need to put some power down. Not that this is likely to be an issue when racing. The Shimano style pedals also have a more defined 'float' (yaw of the foot) than the Eggbeaters, and a more defined point to unclip. Finally, they guide your feet in quite well. I have a few pairs in excess of 8 years old which are still running.
If you are new to clipless pedals in general I'd recommend the M520 as the cheapest way to try them, and from there once you know what you like/don't like you can find pedals that'd suit, as all they should all be compatible with your chosen shoes.
I haven't tried Time or Speedplay so can't comment there.