By far the best android tv app is XBMC for android. Unfortunately it is still in BETA so there are quite a few bugs and video play can be slow. Even with these few problems, XBMC is years ahead of anything else as far as ease of use and expandability.
If you already have an android box and are struggling to get it working, stick in there. It is only a matter of time before the XBMC developers have most of the kinks worked out. Until that time comes, there are a few other choices out there.
A Jailbroken Apple TV or Jailbroken Apple TV 2
These boxes, if set up correctly, can run XBMC up to 720p with very few problems. A lot of plugins are supported and for the most part it works very well. Unfortunately, this had driven up the demand and price.
The Taffy Media Center
Hey, we love our own box here, what can I say? This box is XBMC only and is made to run XBMC and runs it well. Based off of linux and not android, plugins and performance are great! Up to 1080p and smooth transitions, you can’t get much better than this at the moment.