What's your budget?
Right off the bat I'd say Apogee products, even the Duet if you want good quality.
Focusrite iTrack Solo isn't too bad either, 2 in/2 out but a bit clunkier on the go, not a huge piece of kit compared to regular interfaces though.
If you just want one of those interfaces that are more or less just a plug that you attach to your iPad and your guitar, don't expect too much. You'll also want to do a little bit of research before purchasing as some of them are particularly terrible.