best was probably 60-65. i love(d) hellfire ramparts, zangarmarsh, nagrand and terrokar forest. those are some of my favorite zones in all of WoW. end game TBC was also enjoyable.
coming in a close second would have to 80-84 in Cata. or around that range. people probably think im crazy but i really liked mt. hyjal, deepholm, and uldum. unfortunately Cata didnt have a lot of staying power. i felt like it started off really well but about mid way thru our guild quit and have since picked up other games.