Until about 2 weeks ago I would have said "Socket 939" - and I feel that AMD screwed themselves when they dropped 939-pin production. I would have gladly bought a new CPU from them. But since they wanted me to buy a motherboard and memory, I switched back to Intel.
To my surprise, the new Intel C2Ds overclock with extreme ease. Well in my case, I've heard that the C2Q Quad Core doesn't go much over 3.1GHz, where many C2Ds run happily at 3.6GHz. I was able to get my C2Q to 3GHz without trouble. But I'll need to get new case to maintain that speed. Things start to get a good deal warmer at that speed with the Quads.
So my vote clearly goes to Socket 775 now! 