I say a perfect receiver has six HDMI inputs and six 7.1 analog inputs.
It would have a completely discrete mono-block design for both the preamp and amp sections - each channel gets it's own power supply/toroidal transformer.
It would not have a tuner, phono, EQ, tone control, balance control, etc. Bass management would be limited to only speaker size and distance setting.
It would have a FR of 20Hz-20kHz +/- 0.00dB, THD of < 0.0001%, SNR > 140dB, Crosstalk < -140dB, Output Impedance of 0.01 ohms, Input Impedance of 100K ohms, Damping Factor of > 1,000, & Slew Rate of > 100 V/micro-sec.
It would produce 200 watts RMS x 7 into 8 ohms, 400 watts RMS into 4 ohms, and 800 watts RMS into 2 ohms.
But a perfect receiver will be different for everyone, so it will never be made.
It will never exist.