Personally, I got the RetroN 1, which is basically the RetroN 3 without Genesis and SNES slots. I don't know if it has PowerPak compatibility or Game Genie compatibility, but it probably does. The sound is good on every game I have except SMB3, and also it's cheap at only $30. It puts a death grip on your cartridges, though, and the controllers that come with it are sh!t and break really easily.
Actually, the best is the Generation NEX by Messiah Entertainment, but good luck getting that for under $200.