Check out MX Player. It works great with mkv using hardware acceleration. I have to set mine to decode the audio using SW and it works flawleslly.
It also remembers your position in movies so you can go check another app, switch back and you are right where you left off. Quick screen swipes for volume, brightness and seek too.
If you like it, grab the paid version to support the developer to continue to make it better.
If not, check out Dice Player. It also supports HW accel, but I still preferred MX to Dice.