I’ve got more than a thousand “best of” CDs. The others were right when they said that an artist who has recorded for multiple labels won’t have any CDs with all their songs you know, due to which label owns the material. The few and best reissue labels I know of that have been able to cut through the red tape are Rhino, and See For Miles (UK) and Ace (UK). Retrospectives on these labels will usually be from multiple companies. Rhino was a pioneer in this area. Now the company is another division of Time/Warner, and I don’t know if they’re still doing it the way they used to. The founders of Rhino have started another, similar label called “Shout Factory.” Anything on that label will still have the high standard of standardness (ten points to anyone who catches that reference!), but it’ll cost more.
Box sets are for hardcore fans. Previously unreleased bonus tracks and alternate mixes are actually their selling points. If you like an artist enough to buy the box set, they give you these extras because a) you may already have all their albums, and b) the company may have a conscience and not want you to buy ALL of the same tracks again for a higher price. (This isn’t always the case, though.) Plus, it’s new royalties for the artist.
For example, go to the allmusic.com page for Steely Dan. Look at their discography. Count the albums. Then go to the Compilations tab. Yowsah! They have more compilations than they do regular albums! “Gold” came out on vinyl and CD. Then later, they reissued it with a couple of solo Donald Fagen tracks, and called it the “Expanded Edition.” Get “Showbiz Kids” instead.
Some compilations feature the single edit of hit songs, because this is how most people heard them on the radio, and when they bought the 45s. Tracks like this would be otherwise unavailable in digital form. On any given '70s compilation, if it has Three Dog Night’s “Joy To The World,” it’s most likely the mono promo version sent to radio, which used to sell for big bucks. It’s drastically remixed and has different edits than the consumer, stereo single. The latter recording is actually the more scarce one now.
If you want compilations of the real thing, I suggest that you subscribe to Time-Life and get the “AM Gold” and “Sounds Of The Seventies” series. Allmusic is still your best source for information on what’s on which album. For most of them, the track times are listed so you can compare the same track on several albums. If you want the long version, look for the CD with that version on it.
I’m not familiar with Don McLean’s GH, but whoever decided to put an edit of “American Pie” on it should be shot and pissed on!