I didn't go that far.
I've actually never played a MonHun older than 3U.
I'm not as anti-World as many people are. My biggest problem with it is actually it's shitty performance. It has no business running as badly as it does but I guess nips just can't into computers.
That aside, I like the new movesets and I don't mind much of the streamlining. Nobody ever liked having to collect whetstones and if you're not using whetfish instead you're doing it wrong anyway.
I'm not okay with the difficulty, though. It's nothing but super easy all the way through High Rank even. And then you fight Jho who does nothing but throw massive AOE damage around, worse than Brachidios ever did. Now bringing out a DLC that isn't G-Rank sets a nasty example for the future.
As for the others, I liked under water combat and being able to go on an endless hunt in 3U.
I wasn't too much of a fan of the expeditions in (a big game) 4U. I did like the relic weapons you could get, though. Mounting was overdone in that game.
Generations is probably the best of the classic style games. The different styles are a mixed bag. But once you found what style works best for what weapon it's actually pretty cool.
tl;dr: I don't really dislike any MonHun too much but none of them are perfect either.