Cribbage Pro seems to me the best app for serious players. The multiplayer gives you lots of options, e.g. you can play "best of 3," "best of 5," etc. You can set different time limits per turn (15 seconds, 30 seconds, 60 seconds, etc.) and so on.
It is cross-platform, so players using different devices (iPhone, Android, Nook, Kindle Fire) can play each other, which means there are more players available as opponents than other apps, I would guess.
The only other cribbage app I have experience with is Trivial Technologies. I like it and I do think the graphics and user interface are slicker and more attractive on that app. But the multiplayer has fewer options to customize the game (e.g. no best-of series are possible). And the AI doesn't seem as challenging (though it is still pretty good). For instance after hundreds of games (yes, I have a Cribbage problem) on both apps, my win rate against the Trivial Technology expert level is 58% and my win rate against the Cribbage Pro advanced level is 52%. Take that for what it is worth.