I thought 4 was vastly better than 7. 6 I'd put about on 7's level, though. Maybe a little under, since 7 did have some good music while 6 had almost nothing. 5...I'm really weird, but I found 5 to have some strange indescribable charm to it that makes me like it more than I probably should (despite introducing the infuriating Lose Charge mechanic). Maybe it's the severely underrated OST, maybe it's Gravity Man's stage, maybe it's using Super Arrow on a boss, I dunno, but there's something about the game that gives it a special place in my heart.
I would definitely put 7 ahead of 8, though, and quite probably would put 6 ahead of 8 as well.