Mine is Alik'r Desert, which I'm currently in. The rolling dunes dotted with oasis, new animals, the feel of this area is just fantastic, and certainly my favourite so far (the only ones I've not seen yet are Wrothgar and Bangkorai). It just feels like early Tomb Raider (before Lara forgot about raiding tombs) or the Mummy, full of mysterious ancient stuff to explore and the open desert areas are a joy to get around on my trusty cat steed.
PS4 - Europe - Shardaxx - Wood Elf Nightblade - Aldmeri Dominion