Weirdly enough, Metal Gear Solid V.
I had thought of replaying a bit of it (just on my finished save file on PC) a few months back but never did. Then it was the PS+ free game so I downloaded it, and slogged through the hospital bit so it would just be available to play whenever I felt like it.
It turns out I feel like playing it a lot. I've probably played through 2/3rds of the campaign again and a fair number of side ops. It's such a solid game mechanically that it is still a ton of fun even though I've seen almost all of it before.
I'll probably have a lot more old games to judge for this category soon since I'm going back to school and thus am going to be a poor student again. It'll be PS+ stuff and my Steam backlog for the next year or two. And stuff on Switch.