I have tried several and there is not one app that does everything best.
As for departure and arrival info I liked FlightTrack 5 best. For actual following a flight it was less than average: often a plane was way off its actual track and it was always behind in time. The last versions seem not well received in the App store.
Flightradar 24 is one of the best apps for tracking a flight. It covers the most flights and has great view options. Its information on departures and arrivals isn't as detailed as in FlightTrack 5.
Flight+ is also nice and it has a free version so you can try it.
I also tried some other apps but I didn't found them as good as the above three.