Outland for me, for the contrast of destruction/desolation and natural beauty.
Tough choice between Outland and Northrend.... I still get goosebumps when crossing the Dark Portal into Hellfire Peninsula and seeing dat sky...
it's majestic. On the other hand, Northrend feels like ''home'' to me... the music, the scenery, the snow, everything is so beautiful.
I chose Outland, but really, I can't say which one I liked better, and I can't speak for Draenor since I haven't seen nearly enough of it.
Northrend, but I have almost the same excitement for Draenor.
Pandaria after them; quite lower, though.
Outland is the last one (though I started at BC, so I wasn't expecting); didn't realy liked it...
Mainly because i wasn't massive on WoW lore back in the day so Outlands and Northrend I didn't really understand.
Now i'm really excited to see what Draenor was like given how it's portrayed