I have itunes for windows, windows media player, and realplayer. 
Which of the three provides the best quality CD playback?

Hello Mysticsinger

The quality of the playback depends upon the quality of the file you
are playing.  If this file is direct from a CD (or DVD) the sound will
be the best possible available, if it is a music file on your hard
drive it has probably been compressed.

When recording an MP3 (or music file) you can set different
compression settings.  As you compress the file more the sound quality
of the music decreases.  For instance MP3 compression works by
removing some of the frequencies of sound inaudible to the human ear
that may have been available in the original file.

As for the three packages themselves all three play CD's extremely
well but it is a subjective opinion as to which player is "the best". 
The difference in the players output is probably negligible (your
speakers probably cannot pick up the small differences).

The general opinion of Realplayer in the techie-world is low because
of its poor quality general music format (.rm or .ram file) and the
player is notorious for continually asking you to upgrade or buy the
software.

The differences, in my opinion, between iTunes and Windows Media
Player are marginable and much is down to taste.  The techie-world
again will probably frown upon Windows Media Player but this is mainly
due to it being published by Microsoft - and they like to bash any
product by them no matter how good it is!  Windows Media Player is
definitely more developed in terms of the amount of work that has been
done on it whereas iTunes is a relatively new player.

If I had to rank the 3 players I would do it thus: Windows Media
Player, iTunes and then Realplayer.

Incidentally, my music player of choice is WinAmp
(http://www.winamp.com) which is free and has many add-on features.

Some pages you may find of interest:
http://www.xvsxp.com/applications/audio/
http://www.techstuff.ca/archives/442.html
http://www.animemusicvideos.org/guides/avtech/audio2.html
http://computer.howstuffworks.com/mp3.htm
http://entertainment.howstuffworks.com/mp3-player.htm

If you have any further questions please ask for clarification and I
will do all I can to help.

Very helpful answer and I appreciated the personal recommendation also.