Fallout 2 and Fallout 4 are tied in the running for me.
Fallout 2, when I played it back in the day, felt so tonally bizarre. It clearly thought that it was a lot funnier than it actually was, and that really hurt the game. It didn't take itself seriously enough to be enjoyable. While Fallout 1 definitely had plenty of jokes and dark humor, it was still creepy and heady enough that you could get lost in its world. Fallout 2 never gave me that sort of feeling. Or maybe I never got the chance, what with all the constant crashing.
Hated it.
Fallout 4 feels like STALKER-lite in its gameplay (not terrible), and in its presentation - hell, I don't even know, man. It's so bland and milquetoast and strange. Its fixation on settlement building felt completely and totally wrong. Its fixation on the dog companion felt completely and totally wrong. All of it felt wrong. It was tonally discordant and offputting throughout. It did have its moments. Nick Valentine was a good companion (at least in my opinion). Honestly, so was Codsworth. Guess I just have a thing for robots. That, or the person/person(s) responsible for writing the robots did a better job than whoever did the other companions.
There are a ton of franchises I can think of that have a black sheep in them I dread to play, but I'm thinking about Fallout because I recently reinstalled F4, and was shocked by how mehhh it made me feel, right from the start.
Other opinions:
MGS 5 is one of the best stealth games I've ever played, purely from a mechanical standpoint. I've never had so much fun crawling around in dirt. I will say nothing more about this game, however, for the sake of us all. Suffice it to say, it would all be very, very negative.
Dark Souls 2 is an easy game to pick on, but for good reason. It's clunky, it's chunky, it feels strange and floaty, and my god the animations. And the digital 8-way movement. Why did they do that? They didn't in DS1! Why would they do that in DS2?! Maybe to make the movement easier to interpret for online play, a bandwidth-saving measure? IDK, but it was awful. The visual presentation was cheap-looking and wretched. The combat lacked all sense of impact. The sound effects were awful. The weapon degradation was onerous.
I have tried no less than six different times to slog through it, hoping I'd find some joy in its existence. But no. Oh, no. There is none to be found. I could replay DS1, and have replayed DS3 more times than I feel comfortable sharing. But even though there's an entire "Dark Souls" game sitting there, ripe to be explored, I will have to leave it be.
Ah, and Zelda. There were more misses than hits in my opinion. Link to the Past was the game of my childhood, and the original Zelda was my favorite game on the NES. Ocarina of Time is one of the best games ever made, and basically a masterpiece. No game has ever replicated its feel, its sense of wonder, in my opinion. I recognize just how much of a miracle it was they were able to achieve what they did with OoT, given how they were themselves pioneers in the 3d open-world action-RPG-on-console space.
And yet I feel everything after OoT has by some measure been disappointing. Not simply in that OoT achieved such lofty heights that it is hard to match (which is true), but also in that there's nowhere near the same level of quality. For all that can be said of Wind Waker, Twilight Princess, and Skyward Sword (Buy New Batteries, the Game), I feel that Breath of the Wild and Tears of the Kingdom are the biggest disappointments. Save some occasional moments here and there, I felt they had little to do with the franchise as a whole, and felt rather soulless and bland.
There's also the fact that accidentally beating the game feels rather... bad, and a fair bit disappointing. That was, I guess, rather unique.