There aren't a lot of genuinely great mobile games, but Threes! is certainly one of them. What I like most about it is that it actually understands what it means to be a mobile game: Short rounds, no constant focus required and it can be put down at any time since continuing a round is pretty so easy as the complete state of the game is always visible to you.
And for everyone who likes Threes!, I'll also recommend Twinfold. It's a game clearly inspired by Threes but with some dungeon crawling and rogue-like elements added on top. Maybe not as tightly designed as Threes!, but with just as much love and detail put into it and its presentation.