Here it comes.
Pokemon Black and White.
Why? You know why, if you have been following the franchise since it was first released on the Game Boy. Abridged version: something about this game installment/Gen didn't feel right. Hell, it wasn't even released for the 3DS, which was released a few weeks afterwards. Yes, it introduced a lot of neat features, but frankly, most were in disappointing ways, specifically in the regional PokeDex department. I get that they were trying to do an homage to the first Gen and have 150+ exclusive critters, but it didn't work the way they wanted to. This is only scratching the surface, mind you.
The reason why I didn't go for the entire 5th Gen (Black 2 & White 2), is because those two titles are marginally better, only in the sense that they took out a lot of BS from the first installments and added a lot of content from the previous 4 Gens (older Pokemon, past Gym Leaders and Champions, etc.). Also, they're NOT the first sequels in the handheld franchise; that honor belongs to Gen 2, Gold, Silver, & Crystal (I mean seriously; their original titles in Japan were going to be Pocket Monsters 2).
Thank the powers that be that we got X and Y; If we got another mess like Gen 5, I would've just quit the franchise entirely.