In many of my older articles (like this one), I've praised BUZZ Player. ($3.99) and BUZZ Player HD. ($3.99). Up until 4.0.2, which royally messed up the player's MKV playback capabilities, making it one of the worst choices for MKV playback.
(A quick note: note the full stop after the name of both the iPhone and the iPad-specific apps. I've explained the cause for this seemingly amusing name HERE.)
In the meantime, newcomers to the iOS multimedia player scene, nPlayer ($4.99; read my latest review) have become the most important “go to” player for everyone looking for MKV playback and not wanting to play back DTS audio.