I have owned both the Marantz 6200 and the Yamaha 1200. I had a number of defective units in the Marantz line 5200/6200 (5 altogether) so I went with the Yamaha.
What I can tell you is that I prefer the Yamaha for movies, and I prefer the Marantz for music. I can't really tell you if one of the units is really superior, but I think that everything considered, I prefer the Yamaha to the Marantz overall (probably because I watch movies more often than I listen to music).
But, like always, if it's possible you should really try to demo all of them in your system, and let your ears (and your wallet) decide which one is the best fit for you.
BTW, I can't help you out with the Denon, but it should also be a fairly good receiver.
I demoed the Denon 2802 next to a Yammie 1200 and felt that for music the Yammie was warmer and far more musical then the 2802. The speakers used were DefTechs.
Btw, I own a Yamaha RX-V2200 so I may be a bit biased. I can't however say anything about the Marantz but I've heard there have been problems with that line, although supposedly fixed by now. Yamaha on the otherhand is known for it's excellent reliabilty and build quality.