The locations in Witcher 2 were breathtaking. First time I stepped out from that tent and saw those massive trebuchet raining down hell on a distant castle with stunning attention to realism and detail, I was hooked. And from there things only consistently got better, that forest in chapter one is amazing!
Still my favourite area? Hmm... well, corny as it might sound, I really really loved Ashenvale and Stormwind from World of Warcraft. Both locations just absolutely took my breath away, especially as a longtime fan of the Warcraft franchise before WOW came along. That first time stepping into the Valley of Heroes and seeing the statues of all the heroes from Warcraft 2 flanking the entrance to Stormwind, with triumphant music playing in the background, was just so satisfying. And then with Ashenvale... well, just listen to this music!
Exploring the various locations in WOW was probably my favourite passtime while I still played. If nothing else Blizzard have some of the best musicians and level design in the gaming industry, though as of late I question some of their other elements.