That's a tough question, so I'll just list a few of my favorites,
Pokémon Go - because I've been playing Pokémon for 16 years.
Monument Valley - I just wish there were more levels and it was more challenging.
Chameleon Run - while not difficult casually, it becomes incredibly difficult to collect all the stars in a level as they progress.
Infinity Blade - at the time of its release this for me is an excellent example of a what a mobile RPG game should have been. I haven't gotten around to playing the sequels yet.
That's what I can think of at the moment. There are some cool games on iOS, I just don't have as much time to play them anymore.