Hi there, I'm the developer of Yacht Dice Games, a rather modest clone of Yahtzee/Yatzy (both US and Scandinavian rules available). I'm not gonna brag about the app, since there are plenty of yahtzee clones with graphics and interfaces that are just as good or better.
What I will say though is that if you enjoy an actual challenge, Yacht Dice Games has what I think are the best CPU/AI opponents available in any yahtzee game on the App Store. If you play vs 3 CPU players, you are not likely to win even 50% of the time.
Below is the info for the free version, which is ad supported but has almost all the features of the paid version, which you can find here: Yacht Dice Games on the App Store