Roland Sound Canvas? Got to have at least an Air to get this to work from what I hear. I'm still sporting a plain old 2 so have not tried it yet.
ThumbJam is very expressive directly on the iPad interface, but has a bit of latency with MIDI for me. iM1 is wonderful but easily over-taxes the older iPad CPU.
One other that I like is Bismark bs-16i: an eight-part multi-timbral ROMpler with fairly decent sounds and rock solid stability at a reasonable price.