GSP and Silva are both amazing fighters. The two best P4P fighters by a wide margin. However, that being said, I give Silva the nod for #1 simply because he tries to finish fighters. He fights smart (not going to the ground with Maia/Leites) and trys to frustrate grapplers into standing with him, and if they do, he finishes them. If they don't he embarasses them.
The main reason GSP isn't my #1, he lacks either the killer instinct, or the willingness to use it.
Edit: GSP fanboyism is hilarious.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit