Holy Jesus, where do I begin?
3D Sonic games generally would be a good start. Not that they're all awful, but I every time I replay one, the fun I had decreases by a massive amount. For example, I remember loving Sonic Colours but on my last attempted playthrough I stopped playing after the first few levels of Sweet Mountain and Starship Carnival due to sheer indifference to the game.
Medal of Honor: Rising Sun. Hands up if you remember this turd. As a kid, I was in constant fits of laughter from the singleplayer, but I remember enjoying the multiplayer with my brother. After digging up my Xbox and going through the games...there were some casualties, but this game activity tried to make me angry. Horrible level design, ugly fucking graphics (even for a sixth-gen title and ESPECIALLY for an Xbox game), bad AI, hilariously crap animations, the list goes on. Never went back to try the multiplayer. Most of the games were average more than anything else (some Scooby-Doo tie-in, The Incredibles, Blinx the Timesweeper) but Rising Sun was just fucking awful.
The aforementioned aggressively average games that were lacking in fun. They weren't laughably bad, but they weren't very good either. Hell even the games that were fun weren't as good as I remember them being (sorry Simpsons Hit'N'Run).
Finally, Mario Party 4. One of my first gaming experiences, ever. At some point down the line, I ended up with Mario Party 8. I thought "My god this is bad. There's no way the old ones were this shitty, right?" Wrong. Unfortunately I didn't own the game (I played my step-brothers copy), so I had to resort to SGB's Drunk Let's Play. All I saw was the same game I had just dismissed a little while earlier.