As the title suggests, "Reversi" has the most challenging AI of the 3 Reversi/Othello games I've tried so far. Morocco was only able to tie me once on hard (about which I'll not avoid whining about the fact that I was playing half-a**ed due to the lack of challenge) and Othello was a total breeze. Reversi, on the other hand, whoops me completely every time on master difficulty.
As other reviewers have mentioned, other features include saving the game on exit, an available moves display, hints (haven't tried, but the AI is obviously good), a scoreboard to keep track of your record, and some very basic instructions. The undo button is also nice in case you accidentally hit the wrong square. The graphics are basic with animation for flipping pieces and sound effects for moves and flips. In all honesty though, I'm not sure a simple game like Reversi really needs much in the way of presentation. I personally would prefer not having to wait for the pieces flip sometimes. One thing that all of these games lack is the ability to play against another person over the network. I, for one, really hope that this feature is eventually added.
So if you're looking for more of a challenge than the free apps offer, pony up the whopping $2 (and save a penny for a Tootsie Roll) and I don't think you'll be disappointed. And for those who complain about all of these games being impossible, just learn the rules and practice a little, maybe even look up some strategies (there are a few basic ones that I'm sure will get you past the easy difficulties) and you’ll enjoy this great game.