What apps can or cannot run depends on the iPad you have and what iOS (operating system) it can run. Apple iPads and highest iOS can be summarised as follows:-

iOS 5.1.1

The iPad {1} was a good workhorse machine but the iOS has slowed it down so much that almost nothing runs on it anymore. You can run synths on iPad {1} but it can become unstable and just kick out of the app. But all is not lost.

IMPORTANT - If you want to run synths, you must do a full FACTORY RESET (This involves using another computer with iTunes). Things work really well after this.

Next, only install apps which you're going to use. Delete anything you don't need. Most apps won't work anyway.

Another thing; be patient. It is slow to load so don't repeatedly press the screen. Press once and wait for things to load.

Best Synth Apps for iOS 5.1.1:

1) Akai: SynthStation - US$3. Akai SynthStation is a classy 3-Oscillator analog emulation. It has some nice sizzle to it. It's also multi-timbral as a poly synths and 2 mono-synths. The reviews complain about crashing. It's the iPad. Do the Factory Reset. Synthstation is good enough to use solo as a main synth.

2) TakTech: DXi FM Synthesizer - Free. This is a 4-op FM synth a'la TX-81Z with nice graphic interface. A bit tricky to use fingers to do fine edits. No so obvious but you can SAVE your edits. I would use this to layer with another synth.

3) IK Multimedia: iGrand Piano Free - Free. It's not a synth, I know. But you get a pretty decent piano. The end.

4) Novation: Launchkey - Free. Launchkey has several unique presets which are starting points. You move spinning circles to change the timbre of the sound. The "Pads" section has good ethereal patches. It's fun to use. I would use this to layer with another synth.

5) IK Multimedia: SampleTank CS - Free. You only get a handful of samples. The Vibes and Strings are nice and useable.

6) Retronyms: Synth! - US$1. Synth! is based on Casio SK-1 type of beeps and blips. Not for everyone!

Beyond iOS 5.1.1

My ranking is based on sound quality and usefulness. Also, where I've indicated "Free" means I did obtain a free app (not just the one with web-link).

Best Synth Apps:

1) Moog Music: Minimoog Model D - US$8. The classic MiniMoog. You just have to have this one.

2) Korg: Gadget LE - Free version. You get a polysynth called Marseille and a monosynth called Dublin. Korg gives for free a huge selection of good presets covering a wide palette of timbres.

3) AudioKit: FM Player 2 DX Synth - Free version. FM synth giving a massive selection of finely crafted presets. The range of timbres is quite wide; much like the DX7 presets range.

4) IK Multimedia: Syntronik - Free version. You get 25 useable presets spread from 17 classic synth models. Not a huge number but you can split and/or layer 4 of these sounds; which is why it gets a high rank.

5) AudioKit: Synth One - Free version. Subtractive synth with a massive selection of nicely detailed presets. The timbres tend to be more atmospheric and moody.

7) Steinberg: Nanologue - Free. MonoSynth capable of quite powerful sounds. It's good at one type of sound. I would use this for Synth Bass.

8) Yamaha: FM Essential - Free version. 4-op FM synth. Yamaha gives for free only 10 presets; of which only a few are useful (They are Pop Bells, DX100 bass & E.Piano). Still useful for layering.

Anyway, keep on making music. And don't forget to have fun!