My impression is that all of the differing codecs and even bit rates sound ever so slightly different (presumably because they are not lossless and are transcoding from one format to another, so I would say just take some tracks you enjoy, encode at several different test bit rates, put them all on your intended audio gear, listen to the songs over time and see if any particular tracks stand out as particularly good or particularly bad.
Obviously, either keep the original cds or at least a lossless format company if your equipment or listening preferences change over time.