X-Men: Mutant Academy (Crawfish Interactive/Activision, 2000)
1v1 fighter that has great menu music, but then terrible music in-game. Kind of like how it has bad character sprites mixed with great backgrounds, or how there's a surprising variety of moves, but crouching and punching is enough to crush the AI. A game of two halves.
X-Men: Mutant Wars (Avit Inc/Activision, 2000)
Despite the similar name, this has nothing to do with Mutant Academy. It's an incredibly basic brawler where the enemies mostly don't bother to attack and picking up health makes the game freeze for a whole second so it can adjust your XP. On top of all that, Wolverine here only seems to have one move.
X-Men: Wolverine's Rage (Digital Eclipse/Activision, 2001)
A mix of a brawler and platformer that doesn't do either genre well. It has a very basic set of attacking moves and imprecise jumps. Worst of all, the music might as well just be a sequence of random notes.
Xena: Warrior Princess (Titus Software, 2001)
A Zelda-like that might be okay. I'll never know, because it's another entry on today's list with terrible music. There's a backing instrument on every track that almost sounds like someone slowly sending a telegraph.
Xtreme Sports (WayForward/Infogrames, 2000)
A game about entering an extreme sports competition, which in practice means a bunch of different minigames. I played Skyboarding, which has you falling to the ground while collecting arrows to set up combo tricks. You can attempt to perform a combo at any time, but you'll lose all your arrows if your inputs are wrong or you hit an enemy, so there's some push-your-luck elements to it. Had a great time with it, so it's going on the list.
Xtreme Wheels (Spike/Bergsala, 2000)
Bergsala is Nintendo's distributor in Sweden, and Spike would later merge with Chunsoft and create some of my favorite games ever. You certainly wouldn't have been able to tell that from this game, though. It's Excitebike, but everyone is constantly falling off their bike, there are seemingly about 200 riders on the track, and you have a stamina bar that forces you to stop every now and then. Decent music, but not at all fun.
Yakochuu GB (Athena, 1999)
"Most unreadable Japanese font on GBC" is a competitive category, but this visual novel has a good claim to it. It has extremely obnoxious background noises as well, and it keeps those backgrounds around for way too long.
Yar's Revenge (Telegames, 1999)
Someone gave me this as a kid and I still have the original cart. You need to blast through the shield of the enemy base without getting hit by either the slow rectangular homing enemy or the fast dumbfired star enemy, or any number of other hazards on later levels. You can't shoot through the field in the middle of the map, and you need to physically touch the enemy base in order to unlock the missile that can kill it. Once you win, there are several hundred more levels.