Leading on from what Lordnoize said, a PS3 or BluRay player could be a good bet.
Failing that, an Apple TV wouldn't be ideal if you're trying to play stuff from an external device as the latest models only stream stuff over a network from your computer / iOS device.
If you want a device that will play everything you throw at it and are working to a budget, try something like this -
http://www.amazon.co.uk/Designer-Habitat-Nano-3-0-Flashdrives/dp/B005ERNITE/ref=sr_1_1?ie=UTF8&qid=1340989008&sr=8-1Its basic and the interface will probably be pretty naff, but it'll do the job and its cheap. If you want to spend more and get a media player with more features, look up devices such as the WDTV Live, Boxee Box or Popcorn Hour's.