In reply to Matt_79:
On a fine day, Pen y Fan will be swarming with people coming from all directions, and psycho sheep trying to mug you for your sandwiches. Cribyn and Fan y Big will be slightly less busy simply because they're more than an hour's relatively easy walk from the road.
As others have said, if you want quiet, go to Mynydd Du (the Black Mountain, to the west), with its spectacular long escarpment from Fan Hir all the way along to Llyn y Fan Fach.