I've had fun in all the zones, and all the instances so far. Each zone has been unique, with its own challanges, beauty and charm.
The one zone that made me really go wow, they did this excellent is the Un'Goro creator. You get an awesome sense of lushness tropicanna while in there. The colour are vivid and the background sounds of the birds and wildlife just simply draws you into it. The numurous raptors, Giant T-Rex's and stegasaurii give this place an ancient and un-touched feel to it. Adding to the excitement of the area is the fact the town is shared by both the alliance and the hord. There is one particular quest in there that really piqued my interest, the infamous linken quest. The quest has about 13 parts to it and takes you all over the Kalimdor continent, with stops in Feralas, Winterspring, Tanaris and Felwood. This quest alone gained me 80% of level 52 or 53.
As for least favourite area, that would be handsdown, sithilis. It is dreary and it is incomplete, to say the least. Only a handfull quests exist in this area designated as level 55 and higher. At least only a handful existed when I ventured there. So there hasn't been much use for me to go there.