thats a tough one.
Prot is a mixed bag for me. being a tank requires the rest of the team to have at least half a brain cell. if they dont, it by far a terrible experience. if the team is awsome, being prot is too much fun.
As a Ret, having fun is pretty much garenteed. i shread in BG, and as long as i dont match up agaist 1500rated full S3's (power lvling ratings i assume) i do pretty good in arena also. and in pve once people take the time to try me out, you get a good amount of compliments due to everyone expecting a Ret to be terrible.
i think Ret is number one for me. so far, nothing beats getting jumped by a group of Alliance, lets say 5ish, and dropping 2-3 of them before they can wittle me down. all while playing whack-a-mole on the flag waiting for the cavelry.