I'll be honest, I haven't heard the Denon. However, I own the 84 and I love it. I haven't found one thing about it that I don't like. The MCACC auto audio setup works great. Upscaling works great and having 4 HDMI inputs is awesome. I don't have a 1080p tv or HDDVD/BD, but according to other owners, this unit passes 1080p to the TV from the source. There is a known issue with the LFE channel if you use PCM from HDDVD/BD, however, Pioneer will be releasing a firmware update within the next month to address it.
I don't have HDDVD or BD yet, so that doesn't affect me anyway.
Of course when you consider that the list price of the 4806 is 2500 more than the Pioneer, I'd have to see some pretty substancial evidence to spend that extra cash.
I can't really speak for any other brands. I chose the Pioneer as for me it was the best bang for the buck.
