Those free console emulators by that explus alpha(See previous post) Are your best bet on ouya. I instantly imported to my shield tv and both work pretty well, the ouya is good since it can use wii controllers, but the shield has slight performance advantage and even slight better controls. So either way i use those a lot.
I tried ppsspp on it, but power stone works like a bit of a butt compared to shield, i didn't even bother with dream cast cause of the pathehic storage capacity. 6GB is so awful, compared to others, I accidentally bought a PSX emulator, it was okay but epsxe port is godlike especially on shield. I was gonna try it on ouya if i can find some way to get more storage.