Great thread choice. I'll share my favorites, starting with the MIDI performance Apps which have revolutionized my workflow.
SoundPrism Pro:
http://www.soundprism.com/index_pro.php Stunningly simple, yet very complex. Not being a great keyboardist, I've gotten many songs' worth of dope chord changes out of this one. Works great on hardware, too. Here's a video I made showing how to use this as a MicroKorg controller:
PolyChord:
http://www.polychordapp.com I like SoundPrism a little better, but the strum feature on PolyChord makes it well worth it. Here's a video about using PolyChord as a Reason 6 controller:
Genome MIDI Sequencer:
http://www.whitenoiseaudio.com/apps/genome-midi-sequencer/ AMAZING. I've used this to control Ableton, Pure Data, the MPC1000, and NLOG iOS all at once and it does a great job. Great swing features, easy to get around in and fun to use.Only like two bucks more than the Koushion MIDI sequencer,
https://itunes.apple.com/us/app/koushion-midi-step-sequencer/id424454249?mt=8 and Genome eats Koushion for lunch.
One syth app I haven't seen any love for yet is NLOG. I use this to feed signal to my talkbox onstage, and it works and sounds great every time.
https://itunes.apple.com/us/app/nlog-midi-synth/id391268291?mt=8 Super cheap right now and has AudioBus support!
And of course, TouchOSC!
http://hexler.net/software/touchosc The most versatile app on the planet. Must have!
Peace!