This is a difficult subject because there's not one series of releases which really has consistent quality, lack of errors and such.
Here, in my opinion as a bloody American, are my favorite issues of Queen's studio albums (using my collection page as a guide.. you can see that I have 2 or 3 versions of many albums http://members.cox.net/breakfree/cds.htm
) For the purpose of my notes, remember the 2004 card sleeves contain the 2001 remasters, so these are available in standard cases too.
Queen: 1991 Hollywood version. Good quality and includes two worthy bonus tracks (Mad The Swine and Keep Yourself Alive-long lost version). The 2001 remaster isn't very impressive and the high frequencies are too sharp.
Queen II: 2004 card sleeve. Though this one is tough. Nothing wrong with the Hollywood version plus you get 'See What A Fool I've Been' as a bonus, but I do feel the 2001 remaster is superior quality.
Sheer Heart Attack, A Night At The Opera, A Day At The Races, News Of The World, Jazz, The Game: 2004 card sleeves. All sound better to me, and particularly NOTW and The Game really kick ass.
Flash Gordon: I only have the 2004 card sleeve, and only to complete the set, but the quality is fine.
Hot Space: 2004 card sleeve
The Works: 1991 Hollywood version. As Adam stated, beginning with this album, the 2001 remasters are suffering a lack of midrange. As a bonus to getting the Hollywood version, you get the b-side 'I Go Crazy' plus IWTBF in the superior extended mix form, and Radio Ga Ga extended.
A Kind Of Magic: Forgetting what I just said about missing midrange, I'm gonna have to pick the 2004 card sleeve anyway for this one. I prefer it over the Hollywood version for some reason, and definitely over the original 1986 version.
The Miracle: In this case you probably need two versions. LOL. The Hollywood version has a more pleasing sound quality, but they used an alternate mix of 'I Want It All' by mistake, which is nice to have, but you'll probably want the original. They include the 12" mix of Scandal which is superior to the album mix, though a bit too long. The 2001 remaster again is lacking some midrange, but still sounds good enough that to truly have the original album, I prefer it. Perhaps the 1994 U.K. remaster might be a better choice overall, but I haven't reviewed it myself :)
Innuendo: The original issue from 1991. Something is wrong with the 2001 remaster in my opinion. It's inconsistent and some tracks sound really bad.
Made In Heaven: Original issue. Hard to tell a difference here. The 2001 remaster is actually slightly lower in volume (which from looking at the waveform, was achieved by digital volume reduction which sucks).
Having said all of that, if you do start collecting the 2004 card sleeves, you're probably gonna want them all because they're beautiful :) The gatefold albums seem particularly nice (about half of them are gatefolds).
As for The Crown Jewels, it's a great bargain if you can still get a brand new set for $35 on Ebay, but I'd avoid it at a higher price. The sound quality suffers because of 'No-Noise' processing and the quality of the card sleeves is far inferior to the new ones.
Edit: And though I haven't checked them out myself yet, others will probably tell you that the best choices for A Night At The Opera and The Game are the DVD-A issues :)