Honestly, within any given price-range, the quality is about the same between brands.
The general trend is
Yamaha = "Brighter" treble-centric sound. Really ideal for LOUD music.
Onkyo = Generally has a "Warmer" sound, a little less bright, better for movies.
Pioneer = Neutral, is really my favorite, because it's really tweakable either way.
I'd say Yamaha if you're mostly going to be listening to music, or using headphones a lot (their headphone amplifiers are 2nd to none!)
And pioneer for anything else (movies, games, or a combination of everything)
Onkyos are decent, and the cheapest of the bunch, but cannot get bright for the life of them. They've just got a muddier sound to them. Awesome for action movies, or jazz, but lack the "POWER" of a nice crisp high-end.