People hate on 7 because it's the "cool thing to do". When a massive amount of people love something, some people just think it's cool and edgy to bash it. Even if they look like total douchebags doing it without a single real reason.
Some of the reasons 7 is popular:
-Changed RPG settings by being more futuristic and kinda bleak, while most rpgs before it were medieval dungeon crawls with castles, knights, sword/shield/arrows, etc. Before FF7 you didn't see lots of cars, neon lights, skyscrapers, choppers, etc. in these types of games.
-Aerith dies. Usually in a FF game if someone's going to die it's usually the tough guy of the group or an older guy who's lived a long life and ready to go out with a bang, not a young woman who doesn't really see it coming the way it did. It was a total shock and went a completely different direction than the usual for FF.
-The story evolved greatly. It started off as a group of heroes, who were *** terrorists that kill thousands of innocent people no less, fighting against an evil corporation that now controls the entire world. That itself would normally be the entire game, but by the end of the first disc, a new threat arises that effortlessly kills the big badguy so far and gradually works towards destroying the entire planet and everything on it.
-I can't really think of many villains that were physically strong and brutal enough to impale a giant monster on a *** tree and leave it as a sign to follow him. That was probably when people realized that Sephiroth wasn't a traditional villain who may kill some people or turn some people to stone. His murders are usually brutal and would invoke primal senses of fear.
-By now, the superpowered, cold, calm, and calculating, silent, fallen hero turned villain is a cliche. Sephiroth however isn't a cliche since he practically invented the whole archetype.
I'm sure there are tons of other reasons I could touch on, but I don't want it to become too wall of text-y