It's hard to say, The Walking Dead would be a good choice, except I'd fallen out with it by chapter 5 on a blind run, so it would be a bit of a waste of erase.
I want to say FFX, that game is just so full of mystery on your first playthrough, to learn all the shocks about Yuna and Yevon again would be pretty incredible and it's an all time favourite games. But it was one of the first games I actually fell in love with and more than that, but I fell in love with it with two friends and my brother, so I think my first experiences of it are too precious to erase.
So yeah, I'm struggling to think of other games with a strong mystery vibe. Maybe KotoR2 or Planescape. It could be fun to play Pokemon Yellow for the very first time again and relearn all the types etc, but I'd still have memories of Gold so that would screw it all up. Same problem with the Uncharted franchise, as much as I'd like to get the wow again. Also those games are pretty wow without having to be forgotten.
The shock of Katawa Shoujo -Hanako would be lovely to experience for the first time again, but that was an important life lesson and I can't really be doing with forgetting about that.
So probably comes down to KotoR2 or Planescape. I want more to experience KotoR2 for the first time again, but I've put a lot of hours into that game in many many playthroughs and it seems a shame to lose all that. But Planescape has a bad middle section, and if I didn't remember how fantastic the games was and how to just run through the middle section, I might get stalled and have never finished the game.
...I'll go for Uncharted 2. I love the game but I don't have strong memories of individual playthroughs to lose, my only firm memory is playing the game at Christmas and whilst I enjoy that memory, I can happily forget it so that I can reexperience the awe of all the amazing moments in that game