Like SaniOKh, there are certain places from most regions I don't like particularly much. Usually caves.
Kanto: Mt. Moon. It stretches on for ages, has Paras, Rocket Grunts, Zubats, the works. The sole redeeming factor of this place is that you get a fossil at the end.
Johto: Mt. Mortar. I don't remember it particularly well, but I know I hated it every step of the way, generic cave encounters and all. And worst of all, after I fought my way through it, I found that I needed a blank party space to get that stupid Tyrogue from the Karate King, so I had leave, dump someone in the PC, and then COME BACK.
Hoenn: Granite Cave. The typical early-game cave, with the added caveat that you need Flash to navigate it. It's full of unusual and annoying Pokemon, like the ever-tanky Aron, the frighteningly powerful for that point in the game Makuhita and our (not) good old confusion-inducing friend, Zubat. The two version-exclusive resident's aren't too bad, though.
Mind you, I actually like this place a lot better than the others on this list.
Sinnoh: I actually don't remember Sinnoh well enough to figure out what I disliked. Maybe Mt. Coronet...?
Unova: Without a doubt, Twist Mountain. It's long, gruelling and boring. Full of nasty-powerful fighting-type trainers, and a worker with a powerful Excadrill. It really doesn't help that this was where my long-term friend Clarence the Leavanny died in my Black Nuzlocke.
Kalos: Breaking the trend, in Kalos it was not a cavern I hated. It was those wretched riding segments where you go on top of a Mamoswine and a Rhyhorn. Sure, Skiddo and Gogoat were fun, but these other two were slow as molasses, and I hated them for it.