I use iP 3GS second year, i have almost all music apps, but i am most-productive in NanoStudio, FL Studio Mobile is nowhere near in terms of flexibility. iVox is good-sounding realtime vocoder from creators of VirSyn, RjDj/Trippy Replay has got some nice DSP algorithms for realtime processing incoming internal-mic sound. I most like DSP module Eargasm, you can produce endless ambient with it.
Moog's Filtatron offer very solid filter (you can control it with ENV/LFO), delay (also tapped to tempo) and amp (with feedback). Signal path is very rich and clean, i have no problems to create 100 MB wavs, really great.
Nanoloop is nice app, but up to now i created only one sequence with all parts. I found it somewhat limited, although i tried many things during play.