I just bought Onkyo HTIB 580 and the sound is weak. I want to fill up my room. I'm thinking of going with Polk Floors(199 each), Polk rear, Polk Center and a sony(79) sub. However, I'm confused between Yamaha, Pioneer, Onkyo and HK for a receiver.
There are a lot of good receivers out there, and "best" is in the eyes of the beholder. You would probably be pleased with any of the brands you mention (and could include Denon as well), assuming the model you choose has the feature set you need.
That said, while I've had good luck with HK and Onkyo in the past, I got a Pioneer VSX-1015TX for Christmas, and have been overjoyed with it. I don't think it can be beat in the $400 range (street price).
This receiver has resulted in a great improvement in my difficult room, no doubt largely due to it's automatic EQ.
I'm an old hand with HT, stereo, and hi-fi, and have seen enough to not be easily moved by manufacturers' claims. I had thought THX was mostly marketing fluff, but I've really come to appreciate the THX settings, and now use them almost all of the time.
The 1015 seems to be much better built than most receivers in this price range, too.