Morrowind for me. There was a ton of secrets and unique equipment in that game. And me, my brother, and a couple of other guys would talk about all the cool stuff we would find through exploring that world every day. I guess this was before wikis were are big thing so not every unique item was cataloged unless you have the official strategy guide.
2K has been teasing a game with a website called Advent Future. The website seems really freaky as its pretending to be some revolutionary site that will promise a better future for all of humanity by living in a single society, but the site intentionally glitches out with with messages saying the Advent is lying and they plan to destroy us all. The most recent image that might have confirmed that this is a new XCOm is this image, which appears to be an anagram for Vigilo Confido, which was the Latin phrase on the XCOM symbol. If this is all true, color me pumped!
Wow interesting. They might be trying to reboot XCOM Apocalypse's mega city premise.