My personal favorite is Morrowind hands down. Second place is a bit tougher. I'm a huge fan of Skyrim, but I'm not entirely sure that's because of the work Bethesda did on it. I like it because of how many mods there are. On any given play-through I'm sure more of the content I use/experience is from mods than the actual vanilla game. With Morrowind, the mods I use complement or slightly enhance the experience that I have with the vanilla game, not replace it.
I think for second place I'll have to go with Oblivion. I actually have never experienced Oblivion on PC and I still love the game so I suppose that's saying quite a lot. Then again.. at this point it may very well be nostalgia. It's been years since I've been able to play it. Without a doubt Shivering Isles made the whole game worth it, though.
Honorable mention to Fallout: New Vegas. Bethesda had the right idea with Fallout 3, but there wasn't enough content to constitute being on the list. I got bored of it very easily. I think I logged a total of like 90 hours. That's less than I have on XCOM, which is quite disappointing for a supposedly open-ended game. With F:NV I really felt like I was playing what should have been Bethesda's extension of the original Fallout series. Obsidian really outdid themselves with bringing the Fallout 1 and 2 universe into the open world Bethesda styled game. Brilliant use of the engine, really.