The Sims 2 is the one that is the most nostalgic to me. It is what I grew up playing, and I sunk many hours into the game. It's the one I still return to as well. It ran smoothly, looked great, and had a ton of fun features.
However, the Sims 3 is special to me also. It's what I played the most in high school/early college. I loved all of the customization features and there was some amazing CC made for it. This was the game that got me into the sims community. However, I never really play it now...I can't say for sure why. I still have all of the physical copies of each expansion, but it's just not the one I reach for when I want to play sims. I think the Sims 2 just has that nostalgia factor for me that the Sims 3 doesn't.
I never got into Sims 4. I bought it when it first released, played for a few hours, and pretty much immediately went back to Sims 3. I've probably played sims 4 for less than 10 hours. It's a fine game, the character creation is very cool (I remember being super wowed when the create a sim demo launched in what, like 2014?) But it's just not super interesting to me.
I am not as much of an avid simmer as I was when I was younger, but when i'm in the mood to play sims, I always pick sims 2.