In that <$100 price range the Western Digital TV Live (or $90
Live Plus) is the best for watching saved content that I've seen so far. It will play pretty much every music format, DVD rip, and Blu-Ray rip format (1080P and 5.1). And it will play them from a DLNA server or a simple network share or an external hard drive. The only limitation is that it down-samples 24bit flacs to 16bit, but that's a limitation of almost all of the current generation media players. I also like the interface a lot.
It's still a young product category. Some of the more expensive players offer a prettier face, or more online content, or up-conversion but everything at any price is still a compromise and darn little has the flexibility of the WDTV Live.