6 is shaping up to be undeniably the best, as long as they get the roster fully up to snuff. Most of the roster so far is are from design points, aesthetically and mechanically are the best versions of themselves, for the most part and is surprisingly balanced overall.
5 was phucked up on a lot of levels upon release, but ended as a really good game and a great roster. It did about as much bad as it did good almost, with most of the bad being presentation and value and heavily slated at the beginning of the game. It's one of those situations where something bad actually got shined up to being good with work over time. As it stands now, it's over hated and underappreciated.
4 was phenomenal for its time and so far, inarguably one of the most impactful SF games since SF2. Emphasis on...for its time tho. Retrospectively, it didn't age well to me tho at all and feels quite dated and annoying to play after its time (actually started near the tail end of the game for me personally). Super good roster for the most part, but it had janky balance all through its time and interations and a lot of goofy shyt that mars a lot of the good things about it IMO. It brought about a lot of things that hold true for the series and fgc to this day, for better and for worse. As it stands now, it's a lil overrated from the standpoints that folks laud over and conveniently gloss over the flaws of the game.