As tuners go, for all instruments, Cleartune has been around a LONG time, and was even abandoned for a while, but the developer came back with a new version...making a lot of people happy. A lot of people like Cleartune because of its retro look.
That said, Peterson also has an app...iStroboSoft...which a lot of band directors say is better than the actual Peterson tuners...
And the tuner that is universally loved by band directors is Tonal Energy Tuner.
These are all not-ukulele specific, but would of course, work with ukulele.