My preferences are
Kalimdor - Winterspring/Felwood
Eastern Kindoms - Vashj’ir/Dun Morogh/Swamp of Sorrows
Outland - Hellfire Peninsula
Northrend - Grizzly Hills (best music in-game imo)/ Storm Peaks
Pandaria - Dread Wastes/Kun-Lai Summit
Draenor - Frostfire Ridge
Broken Isles - Highmountain and a close second Stormheim
Zandalar - Zuldazar
Kul Tiras - Drustvar
I also find myself going to Deepholm, for absolutely no reason sometimes, just because I really enjoy the look of it!