Morrowind is top without a doubt. The level of detail is completely absurd; it's been out for a decade and people are still finding new things.

The Infinity Engine D&D games are based on, well, D&D, but Baldur's Gate and Planescape: Torment use and develop the Forgotten Realms and ...Planescape settings to tremendous effect.

Fallout is a lot of fun too, more for the aesthetic than anything else. Actually, I have to admit that although I don't regard it as a particularly impressive Fallout game, 3 probably has the best pure aesthetic - crawling around the ruins of DC was pretty damn cool.

I do like the Suikoden setting too. It's a really developed and thought-out setting by JRPG standards.

I have a huge soft spot for Breath of Fire III as well, it's pretty detailed and coherent even if a bit lightweight.