In reply to Ciderslider:
depends really on a few factors:
- what sort of grades you wanna climb
- when you wanna climb (summer, winter...)
- multipitch/singlepitch
general summary here (click on the frames inside the picture to get more detailed information):
http://www.climb-europe.com/RockClimbingSpain.html
I would personally recommend Siurana if you like technical climbing, Margalef if you like pockets
adam