For the CD's Greatest Hits avoid the more recent releases, as an example and personally I feel the Greatest Hits II by Parlophone is superior to that released from 2002 onwards.
If you cannot get hold of the Parlophone from 1991 (possibly the I & II double release in 1994 or Platinum in 2000 is identical) for Greatest Hits II then possibly best most recent CD is the 2002 US Platinum release by Hollywood Records.
I have had quite a few of the Greatest Hits 2 and I have kept the Parlophone 1991 and the Hollywood Records 2002 Platinum (has all three Greatest Hits) - having difficulty getting Parlophone Greatest Hits Platinum 2000 release so not compared it.
But ideally hunt the Palophone UK/Hollywood Records US releases.
The link shows all the Greatest Hits, released for UK and then US for Hollywood Records.
http://www.discogs.com/search/?sort...title=greatest+hits&artist=queen&type=release
http://www.discogs.com/search/?sort...title=greatest+hits&artist=queen&type=release
Here is the one showing releases for Platinum (I, II, and III):
http://www.discogs.com/Queen-Greatest-Hits-I-II-III-The-Platinum-Collection/release/375902 and more overall:
http://www.discogs.com/search/?sort...=Queen&label=&credit=&submitter=&type=release
Discogs is very useful as it provides critical information such as catalogue number, year, label, all this information helps to find the one you want; also take note of RM for remastered when looking at list if it is next to a release.
You can click on the tab on both views to also to switch to Master Release.
I must admit I do find it a bit fascinating hunting down the various release and comparing to find the best release, this I have found applies to nearly all music/artists that spans a couple of decades or more and can definitely be worth researching and tracking down, especially as it is not just modern releases that can have sound quality issues but also the initial digital CD releases when the format first came out and also in the 80s and some very early 90s can be a bit poor.
Cheers
Orb