There really is no one single app that covers all that you're looking for. Cubasis is probably the best place to start. It has MIDI and a good amount of decent sounds to work with, along with automation, but the effects leave a bit to be desired.
You can augment it with some inter app audio effects apps. I recommend the ones from Sugar Bytes, Holderness Media, and Kymatica.
Then as noted by Tarekith a few IAA synths might be good too. I recommend Alchemy Mobile (though some people on the AudioBus forum are reporting that the latest update will give you errors when using with Cubasis), Arturia iSEM, and Nave.
While I'm here recommending iPad apps

here are some unique to iOS that are worth checking out:
1. Korg Gadget - Yeah, no MIDI export, but you get a ton of devices in a multitrack environment, effects automation, and it's on sale now too.
2. Samplr - A really great touch screen sampler to work with, with multiple modes of sampling.
3. Sector - Just got this one myself, it slices up samples with different probabilities and playback styles.
For those last two the app AudioShare is handy to have. It's basically a sample manager that facilitates moving sounds between apps.
If I had to pick one of all these it would be Korg's Gadget. I just think overall it's the best, even if it doesn't have the MIDI export.