Reference CDs, IMO, should meet the following criteria:
1. They are CDs you are very familiar with (you have listened to them many times, on many systems, in mulitple listening environments). You have a good idea of what the music on these CDs sounds like, or should sound like.
2. They are CDs you feel are well-produced (recorded, mixed, mastered). You listen to these CDs and think, damn, that guitar sounds great, or those drums are just perfect, etc.
3. They are CDs that are stylistically similar to the majority of music you record/mix. If you record metal, using folk CDs as a reference wouldn't make much sense.
If a CD meets all of those criteria, it is probably a good reference CD for you. When listening on monitors, you want to listen for the frequency balance (even, not too little/much bass, nothing missing), stereo image (panning), and detail. On good monitors, you will hear things you've never really noticed on normal stereo speakers/headphones. If you are an experienced listener, try and hear how the reverb tails off and listen for edit points, etc. in the recording.
Some classic reference CDs are Pink Floyd's The Dark Side of the Moon and Steely Dan's Aja.