Most of this year's Onkyo and Pioneer models will include support for Dolby's new Atmos object-oriented sound encoding. Denon models from last year won't have that, but they will have Audyssey room correction, which can produce significantly better results than the AccuEQ room correction provided in this year's Onkyo receivers. My understanding is that it's also better than Pioneer's MCACC. For that reason, I'd recommend Denon.
Regardless, all three companies make good equipment. If you haven't already, you should look carefully at the rest of their feature lists to see which best matches what you want.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped