BeatMaker is great. It was the first app I got, the only one I'd happily pay $20 for (okay, except for OmniFocus, but that's a productivity thing). I just got Noise.io, but I haven't messed with it yet.
The two that I use the most (other than BeatMaker) are PaklSound (think TenoriOn), and BtBx, which is like ReBirth for the iPhone (and, thus, awesome!).
I should mention, these are both primarily for messing with music/sounds, not for creating a song.
When it gets a little cheaper, I'll probably grab Bloom, but that's really just for playing with sounds, not a *true* musicians' app.