Tricky. I luckily managed to avoid most truly terrible games as a kid somehow (and the few I did play I recognized were terrible pretty quickly). But if I had to pick something, two games come to mind, for pretty much exact opposite reasons:
-Donkey Kong 64: Pretty much the definition of a bloated collectathon platformer, full of all kinds of redundant currencies, repetitive minigames, etc. I was completely oblivious to it as a kid, but going back to it, well... "WELCOME TO BONUS GAME!" quickly became a phrase that sums up a lot of the game's problems by itself (What even makes these things "bonus games" anyway, beyond being a super-loose, as in, name-only-loose, reference to the DKC bonus rooms? You have to do them to get the Gold Bananas! There's nothing "bonus" about them; they're requirements!). It's definitely a game that had a lot of potential and some interesting ideas, but Rare just got a bit too carried away with this one.
-Pokemon Snap: I loved this game as a kid. But it's absolutely devoid of content, and is extremely short, linear, and scripted on top of that. And unlike, say, many classic NES or arcade games, it didn't even have any obscene difficulty spikes or anything to mask those facts and make it seem longer than it was. So, unless you were completely terrible at it or as a loss to what to do, you probably would be able to finish the whole thing in under like 5 hours max, no problem. And then... that's pretty much it. The Pokemon always behave the same way and the same ones show up and stuff, so there ain't that much more to see. So unless you want to try deciphering how Professor Oak scores photos to get the highest scores possible, there ain't much else there. It's still fun while it lasts. The problem is, it doesn't tend to last... at all. Short and sweet is definitely a thing, but Snap really pushes the limits of that.
Alternatively, thinking about it, I guess another answer for me could be Cruis'n USA. Pretty straightforward racing game; nothing particularly special about it. But god, I loved the music as a kid, and that was enough to keep me playing it for a while.