The best keyboard on ios is Animoog IMO ( requires midi out in-app purchase).
It sends note velocity, poly/channel Aftertouch, and you can assign any cc to most of the controllers, including an xy pad and 3 adsr envelopes.
It's quick and easy to recall different mapping presets, simple to send to a different channel/port.
You can also select from different scales or create your own.
And it even sounds great on it's own.
If you need a step sequencer/arp, then StepPolyArp is probably the best thing out there, follows midi clock.
These 2 apps are the ones I use the most as midi controller on the ipad.
But Magellan is interesting too or ThumbJam which can send pitchwheel data when sliding between notes.