Hands down, World of Warcraft. I played religiously from the original up until Legion (about 12 years) and only started slowing down with the following expack Battle for Azeroth. I still play but it's only casual now. I had 7 max level characters at the end of the previous expack and made the sensible decision to go touch grass.
Following that, the first Mass Effect. I had unlocked every achievement including the "play through stupidhigh% of the game with X character" for every companion. Had a save file for every class. And yes, I let Ashley die on all of them except the one I needed her alive for the play through achievement.