Going purely with gameplay-related elements (at least, in an 'overall' view of things) I have to go with:
Skyrim
Fallout 4
I do mean gameplay here. Not story (or the story's quality). The first time I played Morrowind (my first Elder Scrolls game) was back on my original XBOX, I really liked it, but never finished that version (I later finished it on the PC version with the Tribunal expansion I believe). I enjoyed Morrowind, I liked the game's atmosphere and setting, but I never really liked the gameplay; it was very 'clunky' even at the time.
I did buy the PC version of Oblivion (the same week it was released) and I was very impressed by it at first; it was a honeymoon "OMG this is the best open world video game I've ever played" period. It lasted maybe 2 or 3 weeks, then crashing issues started like mad, forcing me to reinstall multiple times, or in some specific cases lost my saved files (corruption after many saves over just a week). I eventually had to put it on the sidelines and actually "forgot" about it for months, then it went into a year or more. Eventually, the modding scene for the game grew bigger and better, with the Unofficial Patch and such, so I delved into that for the first time (overall Bethesda games modding in general) and got to play a much more stable game; but it was still quite buggy and messy. I never actually finished Oblivion per se; even though I did play a lot overall and completed many quests (probably something like 500+ hours total, maybe more), along with just lots of exploration and modding (modding alone was probably half the total time I ever put into the game). Trying to make the story short here but basically at some point I just lost interest entirely, years passed, and one day I went on YouTube and just watched a pretty straightforward playthrough of the main quests until the end to see it at least one time in my life, and that was it. I never re-played the game ever since. But I DID enjoy my time with it, overall (it was my starting point into Bethesda games modding so there's that).
Fallout 3 I really liked too, similar to Morrowind; I enjoyed the atmosphere and setting more than the gameplay. And just like with Oblivion, Fallout 3 was practically unplayable after a week, multiple crashing issues prevented me from simply playing for more than 15 mins at a time during the first few months after the game came out; until the modding community 'fixed' most of it with the Unofficial Patch (but it still took a long time to get there). I did actually finish Fallout 3, however; but I did also skip a number of side quests here and there. I just wanted to power through the game and see it done to move on to other things. I also modded the game a little bit, but not as much as I did in Oblivion.
Then Skyrim, short story... lots of bugs, couldn't play for months. Then took a long break, came back to the game maybe 2 years after release with mods, became more stable and playable and started to enjoy it a lot. It ended up being my favorite Bethesda game overall. Although, when it comes to 'responsiveness' and overall gameplay I still prefer Fallout 4, simply because long-range attacks of any types in Fallout 4 are much better than anything you try to do from a distance in Skyrim (long range Mage powers or Archery / projectile physics / hitbox sizes and hit detection in Skyrim was atrocious; not to mention the terrible A.I. when using Archery and "stealth" made sneak attacks and overall long-range based characters and build almost impossible for me to ever enjoy). I just want to touch upon one thing I did prefer in Oblivion though (not related to gameplay anyway), and it was - quite ironically - the voice acting (and the few voices associated to some specific NPCs, making them memorable over the years for me). I say ironically because obviously we all know how 'cringe' it was in Oblivion, but it was on a "it's sort of so bad it's good" level (or rather, so bad it's actually genuinely funny and memorable). Those voices and some of the NPC faces are carved into my brain forever. And despite how I prefer Skyrim's gameplay over Oblivion's, Skyrim never truly had much in the form of memorable NPCs or voices save for maybe just a very few select (one of which is probably the Bard in most Inns chanting Ragnar the Red, which I cannot go without in the game if I enter an Inn, it has to happen, he must be heard).
And Fallout 4, specifically, despite what some have said about it in general, was my favorite of their 3D Fallouts. I played Fallout 2 a few years prior to playing Fallout 3 though, and it's still possibly my favorite overall Fallout game in the franchise. But when it comes to 3D Fallouts I'd have to go with Fallout 4 mostly due to the overall gameplay, better projectile physics, making the action parts of the game more enjoyable. However, I think I enjoyed Fallout 3's story and overall atmosphere better than 4. But in the end it was about the gameplay for me. Because really if I want a "good Fallout story" I'll stick with Fallout 2. Now when it comes to Fallout New Vegas I never played it, but I did watch parts of some playthroughs from various YouTubers over the years and it's fine, but nothing as mind blowing as many have made it sound like. The writing was good, but that's probably the only aspect of New Vegas that stood out to me in those videos. Then again, I never watched a full playthrough from start to end. But I don't feel like I'm missing much really.
So yeah, for me, Skyrim for the Elder Scrolls, and Fallout 4.