Hmm, difficult. It would easily be G3.5, but since it's lumped in with G3 I'm going with G2.
What I disliked about G2 was the toys, I don't know they look like they were influenced by Barbie being all slender and such, and have weird heads. I just find the overall design of them bizarre.
I could not stand the repeating of characters, single sided symbols and lack of boys in G3, BUT they did have heaps of characters at first. When I started collecting them (near the end of their run) I found it incredibly difficult to find any pony that did not have a single hint of pink on her, anywhere. Every pony had pink something! I also could not stand the cartoons, they are so annoying and bad. I found them so anyway, but they were made for much younger aged kids, around the toddler mark.
G1 is my favourite gen, only things I really am not too fond of is sea ponies and mer ponies. For some reason I see them like the G2's, kind of bizarre. I loved the cartoons as a kid and have that added nostalgia factor, but as an adult, I like the pilot the most and wish they ran with that design of characters and sort of theme. But ah well.
G4 I enjoy the cartoon, the toys, oh Hasbro what happened? They are so much poorer in quality and just finding one without a flaw IS the 'rare/unique' pony
I don't truly dislike any gens, but for me G2's have always been the creepy slender ponies, still don't dislike them, just find them odd.