The TX-SR875 has the most features, connectivity, and power. It has a higher end scaler as well, making it stand out in the "tech" department above the others. It's pitfall is the heat it produces and some other reported issues, such as popping with HDMI inputs (through speakers).
The Yamaha RX-V663 and RX-V863 are very similar. The RX-V863 adds an additional HDMI input, an additional Optical input and adds an Optical output, it also adds scaling (a lower quality one that you might not want to use anyway) and a phono input. They use the same amplifier section and processing. If the extra connectivity or scaling is needed then the RX-V863 might be worth choosing, otherwise the RX-V663 is a better bet.
The SR-7002 is the most expensive of the bunch and offers the Marantz warranty, which is 3 years I believe as opposed to the typical 1 year on other receivers. The Marantz does not have a scaler but still has all the latest in audio processing as well as the other receivers you mentioned. It's power is going to be comparable to the Yamaha RX-V663 and RX-V863. The Onkyo TX-SR875 is going to have far greater output power which you may or may not need. If you did choose the RX-V663/RX-V863 or Marantz SR-7002 you could always add an amplifier if you wheren't getting enough. Most speakers in a mid-sized room would be fine with the Yamaha or Marantz receivers.
The Yamaha RX-V663 is the least expensive and offers a very good bang for buck. If you don't need scaling or more than 2 HDMI inputs it should fit the bill unless you need lots of power.
