They pretty much go in reverse for me, as in keep getting worse.

1) Mainland
2) Outland
3) Northland

1. Kalimdor/Eastern Kingdoms
2. Northrend
3. Outland

I love the Titan lore in Northrend and how it leaked into Sholazar and the Storm Peaks, and Wrath overall felt immensely satisfying. Outland, not so much, especially considering how gay they managed to make Kael and Illidan look, along with Illidan's encounter lacking the awesomeness that Arthas' fight had. Terrestrial Azeroth has always been on the top though, there's just so much variety and so much to explore and discover, and even if you don't count the Caverns of Time, so many dungeons have awesome moments (SM Cathedral Whitemane/Mograine dialogue, TIMMAH!, having a drunken pirate party on EVC's corpse/boat) that anything is pretty hard pressed to compare, even though dungeon and raid design and philosophy is vastly improving.

1- Outland
2- Northrend
3- Mainland