OoW was by far my FAVORITE expansion. It took the best parts of LDoN dynamic content and static content of normal zones and threw them together really nicely. Epic 1.5/2.0, level cap increase, Leadership AA's, HoTT, and Anguish raids were pretty awesome as well.
I would honestly be fine with any content increase really, LoY adds dyes(Thank god), LDoN adds augs and some "dynamic" content, GoD adds some nice end game content and some decent AAs, but OoW holds a special place in my heart and gets my vote for sure.
OOW was OK bu that's where the level cap increased to 70. With the current 65 cap you can go to GoD which (I realize I'm in the minority here) I liked a lot.