What is the exact make/model of the receiver?
Generally speaking, the best/easiest way of doing things is with the digital connection.
Preferably HDMI if available.
For CDs, optical or coaxial digital audio is as good as it gets.
For HD audio on Blu-ray/SACD/DVD-A, you must use either a HDMI connection which supports it (most do), or you must use 5.1/7.1 analog inputs. Two channel stereo RCA inputs on the receiver can support better audio from these formats as well, but you lose the surround mix. This ONLY applies to HD audio sources, not regular CDs.