HK (Harman Kardon), Denon and Yamaha are probably the most popular receivers on this board. NAD is highly regarded but doesn't seem as well represented, partly because HK and Denon are Axiom's top recommendations.
I'm pretty sure NAD is a Canadian company these days -- used to be a German (or was it English) / Canadian joint venture if I remember correctly. Very nice as well.
Lots of happy Yamaha owners here -- only caveat is that if you think you might upgrade to M80s in the future there seem to be a few more concerns about running some of the Yamahas with 4 ohm speakers.
I don't remember seeing many Sonys here -- general feeling seems to be that the high end models are good but that you can do better than most of their low end and midrange models. Then again, you can buy the Sonys for a good price normally while 15% of the posts on the board involve where to get HK or Denon at a better price
Between HK and Denon it's a tough choice. The HKs are sort of basic feature-wise (but have the most flexible bass management in the business) with a fairly striking look, while the Denons have more rated power*, more features and a more elegant look. Denons seem to be a bit more intuitive to use.
*HK's power numbers are really conservative, though, so it's generally accepted that comparably priced HKs and Denons are pretty close in terms of useable power.
The reality is that it's pretty hard to go wrong with any of them (including Yamaha). I know this sounds awful, but I tell people to buy whichever one they think looks nicest, but check back here before buying to make sure they're not paying too much
