That's a hard one
I can't decide whether I should pick Oblivion or Witcher 3. I absolutely love both of those games and have put hundreds of hours into them. I would say that Oblivion as my first Elder Scrolls game and also one of the first RPGs I got really into would be definitley my #1 choice in terms of nostalgia and immersion but in terms of story and gameplay I have to pick Witcher 3. I played hundreds of RPGs and many of them were very good but those two are the ones that stuck with me the most. These are two of my favourite games of all time and also two of the only games that I can just pick up and play for hours and hours any day so I would put both of them at my number 1.