Vash'jir sure! I love the Hyjal lore and have been excited for the zones release since vanilla, but Vash'jir is just well... epic. It is significantly different from anything we have seen before and is a good representation of the radical changes that are to come with Cataclysm. While I will no doubt be doing both zones, I will definstely be going to VJ first.
Ironically enough the zone that has been dubbed as a "loregasm" will probably end up being blazed through by most players do to its apparently faster leveling. I will therefor wait to do Hyjal so that I can fully enjoy the multiple "loregasms" it has to offer.