I like 7 and enjoyed 8 but I never finished a run (it's a long ass game). I played a few chapters of 6 but not enough to form an opinion. I think if I went back, I'd probably like all of them though I might find 8 a bit boring/redundant because it's rather easy. I'm curious about 6's hard mode, I heard it was kind of bullshit but in an interesting way.
Also Roy being dead weight in his own game kind of sucks because he's based in Smash (granted, he's actually pretty shit in Melee, but he's cool)