Definitely Morrowind for me.
[Now the rest is most likely "TL;DR", and the above already answers the main question so you guys feel free to just ignore what's below]
I'd say Skyrim was quite impressive (I don't play it anymore and it's not even installed but that's mainly because it kept crashing all the time), although still not as much as Morrowind was back in the day. When Skyrim was released it did not feel like a "breakthrough" and that's not because it needed to be that either but just saying that Morrowind was just blowing things up out of proportion for its time. I think that Skyrim is still good though, on its own, and certainly better than Oblivion (in my opinion of course).
I'd say that Skyrim is my third favorite Bethesda game so far since I actually enjoyed Fallout 3 more (along with the DLCs). I was "somewhat" addicted to Skyrim for probably two or three weeks, then the dust (from the hype I had) settled down and I literally stopped playing overnight, one day I was playing, the next day I uninstalled it and forgot about it. Not that I "hate" Skyrim, I just don't feel compelled to play it and a LOT of changes have been made to the game since then because I put it aside before I think the second or third patch for it was released. I don't remember which it was exactly, but it was that patch that allowed the executable to be large address aware with that 4GB of memory access thing instead of 2GB... and I know that patch is old so you get an idea of about how long I actually played Skyrim (and really I haven't played AT ALL since then, zero seconds of it).
I admit though, if it wasn't for the constant every 1-hour crashing (no exaggeration) I would have played it more but that just completely burned through my patience and since I've had that problem with Bethesda games since Morrowind (not on XBOX, but my PC copy kept crashing too) and since I quit modding Oblivion due to the game crashing whilst even just trying to test my mods and, again, since the exact same thing happened for Fallout 3 I thought "this is the last time I'm going to endure it" and for once I held to my thoughts to this day and don't regret a second of it. It's a bit of a rant I guess, but also a reality. I know many would say that THEIR Skyrim, or their Oblivion or their Fallout 3 never crashed or froze a SINGLE time, ever... well you know what? Good for you and I'm jealous, because those games demand to be played and I love open-ended worlds in video gaming.
But, with all this said, the point is that when Morrowind got out it felt (and I believe was actually) ahead of its time, at least from my perception of it for the "open-ended" genre. That is similarly to say... Half-Life for the FPS genre was ahead of its time, I think that Morrowind was one of those games. In fact even Oblivion was merely impressive graphically, and I thought the A.I. was also good until not even a day later I realized that seeing NPCs staring at walls all the time and hearing their senseless "conversations" (more like 2 or 3 sentences followed by sniffing and "good day") every specific minutes was just a bunch of scripts without soul nor passion put into them by the devs. And by the way can anyone recall the A.I. demo shown before the game was released with that woman and her dog in a house that supposedly showed how unpredictable, dynamic and "intelligent" Oblivion's A.I. would be, versus how it actually turned up? So anyway, the scale or even complexity of Cyrodiil didn't seem all that "better" than Morrowind's Vvardenfell.
Ironically enough though I ended up modding for Oblivion but not Morrowind, but that's because I had bought Morrowind for the XBOX and never owned it for the PC until many years later (bought it on sales and basically mostly out of nostalgia). Today, however, I don't think I would be able to play Morrowind again since outside of the constant crashing I know I'd get I just don't feel that interested in Bethesda games overall anymore. I think that if I wanted an open-world game now I'd probably have to look for something else along the lines of Far Cry 3, Just Cause 2, Borderlands (which I own, and the sequel too) or maybe something like the new Tomb Raider. I think I'm done with the Elder Scrolls franchise unless they release one that would end up being linear, with actual characters I'd care about and a story with good voice acting perhaps even in the same veins of Dragon Age or Mass Effect. That is I.E. getting a protagonist, with of course some ways to customize the appearance and select the gender, or even the race, then would get a team as the story progresses, of memorable characters (not too many like in ME2 though, that was too much), but with a nicely crafted story with a solid 30 to 40 hours or play and that would be great for the franchise, in my opinion (I'd really get back into the franchise if that happened).
As far as Elder Scrolls Online is concerned I'm just saying "pass" right now.
So anyway, bottom line, Morrowind was the best for me.