Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
Choosing your synth app (only one)
It's easy to post your favorite synths , but just think for a moment (or more!) . On your iPad you could only have one synth app , not two , only one . I am not talking about all-in-one apps like Caustic, Gadget etc , but standalone IAA, AU synths.
I say Synthmaster, not 100% sure because it's not AU ,but I like the ready to play ,high quality sounds.