1) a. Jon Jones
b. GSP
2) Jose Aldo
3) Anderson Silva
4) Ben Henderson
For the top spot, Jones or GSP are the most viable options. I'd give to Jones because he's looked virtually unbeatable [other than that Belfort armbar attempt].
Next is GSP. He hasn't lost in over 6 years, in arguably the toughest and deepest division in the UFC. However, his performances against Diaz, Shields, Hardy and Koscheck leave a lot to be desired. It's sad because he has the skillset and athleticism to put a beating on his opponents.
Moving on, I believe that Aldo is the next p4p best fighter. He's unbeaten since 2005 and has never really come close to losing. He beat Edgar worse than Ben Henderson did, which is a great feather in his cap. The knock against Aldo is that for a good period of time, his division was mediocre [at best]. However, it has gotten a lot stronger in the past year and a half. Also, his performance against Hominick and Florian weren't that great.
Anderson is still one of the best and most feared fighters in the world. I wouldn't put him above Jones, GSP or Aldo because they are all on very impressive winning streaks.
Bendo rounds out the top 5. He's beaten a lot of great fighters, in a very tough division. For the most part, he puts a beating on his opponents [Diaz, Guida, Miller, etc.]. There are, however, times when he has a tendency to cruise to victory.