I've not played a lot of bad games in general (except Bioshock Infinite but thats irrelevant), certainly not older ones, but if I was to pick one out then it'd have to be Castlevania: The Adventure.
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I can forgive a game being clunky or any of that stuff and I think a lot of modern gamers are a bit spoiled in how little roughness they're able to put up with, but I really think this game sucks.
It's exceptionally dull, it's ugly and none of the areas look any good. Of course it's a Gameboy game, so there is a lot of restrictions on what they could do, but the old pokemon games are fun and they were on that same handheld.
Not only that, the game just doesn't really feel good to play either, going from Castlevania 2: Simon's Quest, a very overhated game in my opinion to this was such a drop in playability and fun that if it wasn't like 2 hours long I would've dropped it.
Thankfully the sequel, Belmont's Revenge is actually genuinely really good and fun.