Here are some of the games I have enjoyed most so far on my Ouya, listed in no particular order. What about you guys? Let's compare lists.
Rose & Time
League of Evil
Hidden in Plain Sight
Also, haven't had time to get into this one but it looks pretty cool in the tutorial level:
And though I enjoyed Towerfall, I just haven't been able to bring myself to spend $15 on the full game.