I say 7
The ranking system was screwed up on the first level. I played that a few times trying to get an A for the first level, but the best I could manage was a B (and I got #3 on the high scores all the same... call me Superman). But it did not directly depend on how many points I had. Once I got 2900 which was an F... and a little lower than that was a B. What determines what rank you get?
This may bump me down a few notches on the high scores to tell people this, but the best weapon for the last level is the note with the dot on it. It kills lines of tubas like mad. The triple note is more impressive looking but not as good.