The
tetsuki app for Pandanet is pretty good on tablets and larger phones. It is available in the usual app stores for android and iOS as far as I know.
Go Quest is really nice if you like to play 9x9 or 13x13 because it has tens of thousand of users that play those sizes. It is rather nice on phones because you don't have to zoom in to make moves on these smaller boards. You can also play 19x19, during daytime in Asia, with rather short time controls. It should be available in the usual app stores in android and iOS.
There is also the
kibitz Go server and you can pay a small fee to play on android and probably iOS too. Maybe there is some free option too, I don't know.