If you're going to be doing lots of quad-emphasizing lifts it's not a bad idea to do a bit of hamstring work too (and having strong hams may not be quite as important as strong quads on the bike, but it certainly doesn't hurt). So, I recommend deadlifts. My all-time favorite lift. Makes me feel so strong .

I also like plain old squats, with a barbell.

For both exercises, heavy weights/low reps are best for building power. Careful of your back on the deadlifts, you need to lock your core muscles to keep your back flat (hips tilted forward). Arching your back (in a "cat back" position, with the hips tilting back) can lead to injury.