I'm looking for the best HD streamer (music, photos, internet radio, HD video in mpeg-4, etc) that is compatible with a Mac. Any recommendations/suggestions?
I don't think you can get it all in one spot. I personally use my mac pro to send stuff to other boxes and the Apple TV, but I also mix stuff up with the PS3 and Xbox360. Add Hulu and Boxee into the mix plus other internet radio, Amazon Streams etc and it's a total melange....
I personally use an Apple TV because it's easy and has a great Interface. I have a 46" Sony Bravia, so the current ATV's limitation of 720p is fine for me, Movies I encode to that resolution look fantastic on the set. If you require 1080p then the WDTV is a good consideration, it's UI isn't fancy, but it will play pretty much anything you throw at it including 1080p blu ray rips without a hiccup.
Do you currently have a TivoHD or Xbox360? There are utilities that use them as interfaces to your Mac's media. I've not tried HD movies with either (I now use the 360 for music), but you might be able to get going for little or no additional cost.