Buying apps is not easy.  Some are amazing, many are wastes of your hard earned cash. Here is a list (in no particular order) of my favorite iOS apps for iPhone and iPad.  Of the tons of apps I have purchased and used, these ones get my stamp of approval.


Conductr Ableton Live Controller

Conductr is a dedicated Ableton Live controller, designed for live performances.  You can set up your own custom controls to manipulate pretty much any parameter in Live.  It also has a really cool "no-looking" gestural mode, that allows you to manipulate controls without looking at the screen.  Check out my artist profile on their page.


AudioBus -iPad and iPhone

This app allows you to send audio from one app to another.  A complete game changer!  A must for any serious iOS musician.


Sonic Logic

This is an excellent Midi Controller app.  You can create your own custom midi layouts right on the iPad.  Check out my full review here.


Animoog for iPhone Animoog for iPad

It's a Moog and it sounds like it.  One of the best synth apps on iOS, period.


iMini Synth

iMini is a recreation of a 1971 Moog MiniMoog.  It's got a great sound and some extra features not possible on the original, such as polyphony.  Definitely a must have synth.


Beat Surfing

An amazing midi controller that lets you create and edit control surfaces right inside the app.  Something different, this is what iOS apps should be all about!  This app allows for some insane creative opportunities, I can't recommend it enough.


Borderlands Granular

An interesting and fun granular sampler.  You'll definitely get some cool sounds and the interface is really nice.


Samplr

If you asked me to show you one app, Samplr is probably the one.  This is an OP-1 inspired sampler.  It's an app designed for touch screen, not to emulate hardware.  It's unique and fun.  It's probably my favorite app for playing back samples.  I just can't wait until you can record samples right into the app!  A true classic must have!


LivKontrol

LivKontrol is an Ableton Live controller for your iPad. It allows you to launch clips, compose and edit MIDI clips, and mix your tracks.  It is free and allows you to purchase just the features you will need. 


FunkBox Drum Machine

The FunkBox Drum Machine has a bunch of classic drum machine sounds in a really nice interface that plays well with other apps.  You can mix and match kits to create your own custom drums.  A pleasure to use.


Bebot - Robot Synth

This is a pretty simple but very useful synth.  It's cheap and fun, cool graphics.


Cassini Synth

This is one of iOS's super synths.  Lots of FX and Oscillators, Modulations, Envelops, you name it.  Cassini goes super deep and sounds incredible.


Korg Gadget

This app is amazing. It's an all in one song creation app, with loads of really cool instruments and effects.  Best of all it has an Ableton Live export feature which converts your work into clips in Live's session view!


Korg iPolySix

This is a great emulation of the Korg PolySix and it has a pretty well developed sequencer and drum synth.  Analog and 80s!


Chordion

This is a cool midi controller/synth that works in an interesting way.  You pick the chord and "strum" keys to create melodies.  It'll get you playing in ways you normally wouldn't.


DM 1 Drum Machine

This is my favorite step sequencer-style drum machine.  There are tons of great drum kits, it is easy to program, has cool effects, and supports WIST.


Figure - Propellerhead

A great app for quickly creating grooves and melodies.  Very intuitive and fun.  It's got the same audio engine as Reason.  Each update adds some cool new features.  Figure gets a lot of use on my iPhone.


Filtatron by Moog

It's a Moog filter!  Send audio to it and give it that Moog sound.


Flail 

This is an oddball synth, but one I use all the time. Flail offers a really unique musical interface. You basically touch the colorful mountains to make sounds and arpeggiated melodies. It's really easy to alter the sounds and add effects.  Apparently, it can be used to jam along with Will Smith songs; but I've never tried this feature out... Otherwise, it's one of my favorite synths!


Robotic Drums

This is a great drum sequencing app, but what makes it special is its probability sequencer.  Your loops will never be performed exactly the same way. You can program each hit to stay exactly the same, or have occasional or totally random variations.  Very cool with MIDI out!


GarageBand - Apple

One of the great iOS DAWs.  Its got multi-tracking, amazing instruments with super expressive control, a guitar amp, and syncs up with your desktop GarageBand. Plus it's super cheap.   It's a must have.


Genome MIDI Sequencer

Genome is an amazing piano-roll-style midi sequencer.  It creates no audio but can send midi to other apps and over wifi or wired connections.  It is elegant and smooth.  Love it!


Impaktor - The drum synthesizer

Bang on the desk and Impaktor turns it in to percussion sounds.  It's amazing.  Simplicity and functionality come together beautifully.  This is a must have.


ImproVox 

Create harmonies, auto-tune, and add effects to your voice in real time with the swipe of a finer.  This app is great for quickly making super lush background vocals.  I use it often.


KORG iMS-20 - KORG INC.

This is a sick reproduction of the MS-20.  Patch cables, sequencers, drum sounds, the ability to record patterns... This thing is the real deal.  I used it for a few Ableton Racks here.  A lot like the Nintendo DS Korg DS-10 cartridge.


KORG iELECTRIBE - KORG INC.

A pretty solid virtual Korg Electribe.  If you like that workflow, you will like this.


Korg iKaossilator 

I love this app!  It's really fun and easy to use and there are a ton of sounds.  It's a quick way to come up with new patterns and sequences that can be exported, and it's also just a really cool synth/ drum machine. And now you can export your work to an Ableton Live Project!  Your work will be transformed in to clips in Live's session view!


Koushion MIDI Step Sequencer 

Koushion is a great midi step sequencer, I don't know why I never hear about it.  It sends out midi over wifi or wired.  I use it to send midi to Ableton for drum programming.  One of my favorites.


Lemur

This is one of great successes of iOS.  Build your own midi controllers that are out of this world.  Awesome physics engine, and great response.  If you want something out of this world, this is it.  The only catch, it's expensive (I don't regret it though!)


Magellan 

One of the great iOS synths.  I can't think of anything it can't do. It even lets you stack two synths together.  If you want a serious synth, this is one of them.


MIDI Guitar

Turn your guitar into a midi controller.  I think this app is pretty amazing, sure there is some latency and sometimes the note tracking isn't perfect, but it's pretty damn cool to play synths or drums with a guitar!


Effectrix

This is a great sequencer style multi FX app.  If you want to glitch things up, Effectrix is your ticket.  It accepts live input as well as AudioBus


Echo Pad

Echo pad is a multi FX app that is fully compatible with AudioBus.  It has a great XY interface and can accept live inputs.  Very versatile and very fun!


RgbSoundPro

A weird sampler, sequencer type thing.  It's got some really neat, glitchy type sounds built in, which I think justify the purchase.  It's great fun to mess around with.


SoundPrism Pro 

I love this app.  If you want a cool alternative to the traditional keyboard, this app is it.  It has its own sounds which are nice, plus ones you can buy.  But the best part is it sends midi too.  I really enjoy controlling Ableton and my OP-1 with this app.


Sunrizer synth

One of the iOS synth masterpieces.  It's an amazing sounding synth with seemingly endless control and possibilities.  It has a really cool blending feature that lets you combine two different sounds.  It's a must have.


SynthPad 

SynthPad is a simple iPhone synth.  I really like the sounds though.  It's great for some video game type sounds and goes surprisingly deep.  I even made an Ableton Rack with it.  Plus, maybe the best part, only $1.99.


TNR-i

I always thought the Tenori was cool.  Now I have it on my iPad! What's really cool is that it sends out midi, so you can use the 16x16 grid to control hardware and software.  I think this is a great iPad app, easy to use and fun!


touchAble

This is the ultimate Ableton controller.  It syncs right up with Live and gives you control over almost everything.  If you want complete control over Live then this is your best bet.  Amazing!


triqtraq - jam sequencer 

If you want a different kind of pattern builder, TriqTraq is certainly that.  It has a simple interface, yet goes deep.  I would compare it to Figure by Propellerheads, in how its quick and fast to create melodic and rhythmic patterns.  Very fun!


DrumJam - Sequencer and Performance Tool

DrumJam is an amazing app that lets you slide your finger across the screen to create beats.  The samples sound great and it is all midi clock sync-able!


Bloom HD

Bloom is an endless generative app created by Brian Eno.  It's great for creating ambient music.  I like to jump to Bloom during a live set, to create a mellow transitional track in between songs.

That's it for now, check back, I will update often.  Did I miss anything?  Let me know of your favorites in the comments!