Personally, I voted for (by chronological order): Pearl Islands, Cook Islands, China, FvF, One World.
Pearl Islands because of the intro (this is a huge part in my decision for fave seasons XD), outcast twist, pirates theme (its execution), the winner.
Cook Islands because of the intro, location, the controversial tribe segregation, the inspiring Aitu 4, awesome players with awesome characters.
China because of the intro, location, cultural theme, drama, strategies, the players, first season I watched.
Micronesia because of the intro (which is my favorite), the overall theme was epic, all the blindsides, the players.
One World because of the intro (not sure if it was mainly how we had to wait until the season finale to see its fullest that made me boost this up, I hope not =P), the theme (the one beach thing, imo, was a good twist that gave light to new types of drama that had not been seen previously), controversial and inspiring characters, the winner's awesome gameplay/manipulation.
It's pretty hard for me to have a more personal liking for the first few seasons because I started watching late, but Amazon and Africa would've been next on my list.
I would say my ultimate favorite is Micronesia as to me it is one of the best in all aspects of my criteria. But that's just my opinion! =)