I've only been on three holidays in my life (Hafan Y Mor in Pwhelli, Wales; Disneyland Florida, and Corfu, Greece). Out of those three... I'd have to say Disney was my least favourite.
Don't get me wrong, I love Disney. What made it my least favourite was that I'm not really a rollercoaster fan, and I went with a family who absolutely love them so I spent a lot of time bag-watching. I also spent a lot of time going hungry because they all ate at sporadic times and I had no idea when I'd next eat (to the point where I deliberately skipped a ride I wanted to go on in favour of food). I also got horribly home-sick after the first week. I also caught a cold on the third day since I'm not used to the temperature changes (extremely hot outdoors to extreme air-conditioned indoors) and then caught another cold on the day we were flying back, so I was ill for three weeks in total. I also felt left out by my friends half the time, even when I made a concerted effort to try and join in. I was desperately trying one night, stopped to look at a shelf in a shop, turned around and they were gone... and they hadn't even noticed I was missing. So I sat with my friend's dad for a while since he'd recently had a heart attack and was feeling fatigued. They also decided to go and get a photo with Mickey Mouse without me on a day where we decided to split up, and I chose to stay in the hotel to relax. If they'd told me that's where they were going, I would have gone. That was my sole Disney dream ever since I was a kid, to have a hug from Mickey Mouse.
Hafan Y Mor was my favourite. Beautiful scenery, and peaceful, even if my younger stepbrother decided I was the best thing since sliced bread and was stuck to me (almost literally) for the entire holiday.
So in general my least favourite places would be anywhere active and crowded. I like to relax and chill and soak up the atmosphere. ^^