Kingdom Hearts Re:Coded
Worst game I've ever played was Kingdom Hearts Re:Coded. It didn't look very good, especially coming right after Birth by Sleep and it was remade from the original cell phone game, meaning it had a very shaky basis to begin with. The story was fairly pointless and it didn't help that I'd already seen all of the important scenes before I played the game--to be fair to me, I didn't think the game would ever come out in any capacity I could play (since the game only worked on a certain model of cell phone in Japan before its DS remake). So even the crappy plot was completely unoriginal for me. Literally the only reason to play this game is for the secret scene at the end which explains what's going on in KH3D but I didn't even get that in-game because I didn't meet the requirements. I just watched it on Youtube and wallowed in disappointment.
I will admit that the boss battles were interesting. Each one generally ended up as a different genre. Like, the boss fight for Traverse Town was a sidescrolling platformer, the boss fight in Wonderland was a shootemup (I think?), and so on. But it was really gimmicky and it made all the other fights in the game really pointless and grindy because you weren't even levelling/powering yourself up for the sake of proving yourself to a boss because the boss battles were fought differently. (I think. I've taken considerable effort to forget about this game.) The neon/grid areas of the data world and their enemies were ridiculously repetitive and not fun in the slightest. I honestly don't remember a whole lot of the game beyond what I've mentioned so it wasn't memorable at all, either.
It's a game where, had it continued to be a one-off cell phone game for all of its existence, everything about it could be forgiven, but as a stand-alone DS game trying to live up to the standard that Days and BBS had set as far as spin-off games in the series, it fell so short. It might not be a bad game compared to some other games mentioned in this thread but it was terrible compared to everything else in its series even to this day.
Bit Trip Runner
The only other game that makes me livid to think about because I disliked it so much is Bit Trip Runner. It was a reaction memorization game masquerading as a rhythm game. The game looks great, but that's about all that I can praise about it. It pretends to be rhythm based but I'd found that I played just as well with the sound off compared to on and the jumps/dodges/blocks didn't match up to the beat very often anyway. The music (and the beat, iirc) isn't the same every time you play a level, either, so I don't know where they get off calling it a rhythm game. The game was so hectic and chaotic in what it wanted you to do that it often felt like it was punishing you for learning your cues.
For example, there are these little trampolines you can jump on eventually that boost you up a little bit. The first few times you come across it, they put points or a "power up" (just adds more instruments/melody to the bgm) in a position where you'll only grab it if you jump from the trampoline. Alright, so you learn to jump on the trampolines whenever you see them... except eventually when you jump ON the trampolines instead of OVER them, you get launched into a hole. Sometimes, they mix these up even on the same level so you literally have no choice but to mess up once and then memorize how they want you to tackle each one. It happens with many of the different obstacles, too, so it doesn't feel like you're getting better at the game or learning it as you go along. It's not like you can when game constants are never actually constant.
The icing on the cake is that the game is designed to be unforgiving. You mess up even once, whether you're at the very beginning of a track or just before the end, it sends you back to the very beginning. It's almost impossible to get through something on your first try because it throws so many curve balls at you. Every time you start a new level, you practically have to completely untrain all your reactions. It wouldn't be so bad if you checkpointed or something every time you grabbed one of the four power ups (or just reached a certain point in the level), but you start from the BEGINNING. So longer levels (some get up to 2 or 3 minutes, I think) become agonizing as you end up having to memorize every single section of the song and you have to get the timing almost exactly right each time because if you don't, you're gonna waste that whole 2 minutes over again until you get it PERFECT every time. Changing the difficulty from Normal/Hard down to Easy doesn't help either--literally all that does is take away the points you get as you go through the level and all those do is give you bonus levels for getting all of them. If anything, taking them away in Easy mode makes it more confusing because you no longer have hints ahead of you showing you the way you should be going through the level. (It makes the screen look a little less cluttered but it's not like it's that much easier.)
I'm not very good at platforms--I won't try to hide that fact at all--so already I went into the game at a disadvantage but there was just no leeway whatsoever in this game and I don't think I've ever played something so frustrating in my life. Even finishing a level that's taken forever never felt satisfying: I'd just feel angry with myself for taking so long to memorize the exact timing and progression of every button press. I'd be okay with it if that's how they marketed the game (not that anyone would want it if they did) but the fact that they push it as a rhythm game when it really doesn't work as one just makes it all the more frustrating.
/whine whine whine