I think SF5 and SF6 both suck.
SF4 was peak. Yeah, the game had some issues, but the main complaints were... comeback mechanics and 1-frame links.
Comeback mechanics are now so ubiquitous that people don't even complain about them anymore, and 1-frame links are a minor execution issue.
SF4 was the peak of the series, even better than the previous games (also good but had issues of their own, roll cancels and parries change the game too much). It was the peak of the competitive scene too, the irony is that new games are made to be "fun to watch" yet are less fun to watch than SF4 was.