[rohall]

The easiest one I've used is Figure (http://www.propellerheads.se/products/figure/). You'll probably want something on your desktop/laptop to pull everything together. Feel free to reach out if you'd like help with workflow.

I made this using mostly Figure and Garageband (to give you an idea of what can be done): http://rickohalloran.bandcamp.com/

[Tloewald]

+1 for Figure.

Looptastic HD is quite nice for putting something together with almost no effort.