This is a tough question because it's like asking "What's the most forgettable game you've ever played?" I can remember some of the best and worst games I've ever played, but mediocre games are explicitly not interesting.

That said, the first one that came to mind for me was Starshot: Space Circus Fever for N64. It's just a very generic late-'90s collectathon platformer. It's hard to be mad at it, because it's not terrible or anything, there's just no reason to play it. If you've got an N64, there's Mario, Banjo, Rayman, even B- and C-tier stuff like Gex and Chameleon Twist. There's hidden gems like Space Station Silicon Valley or Rocket: Robot on Wheels.

That last one is the only reason I played Starshot, I saw it clearanced at a used game store and was like "Oh yeah, I remember hearing this game was good," but it turned out I was thinking of Rocket. That game actually is good, while Starshot is just fine.