Deus Ex.
I've done a minimum of one playthrough each year (sometimes more... several more) for the last decade. Started when I was still in middle school. It never gets boring. I've done completionist runs, speedruns, sadist runs, pacifist runs, exploration runs... you name it, I've gone through it. I still enjoy the hell out of it, and love the different combinations of ways to approach objectives.
I've also dropped a good 200+ hours in Baldur's Gate II across 3 playthroughs. It's probably the only game that I finished, then restarted immediately for a completionist run. It's that good.