How are 'Best Of' compilation albums mastered?

From my experience the bulk of "best of' compilations go for quantity over quality. Too many tracks per side, thus compressed. My general rule of thumb is - at least on vinyl - stay away from artist comps.
Paul

Compilations usually good on cd and lesser quality than originals on vinyl historically.   Maybe new vinyl compilations are better but yes quantity usually reduces quality with vinyl in that more modulation in groves means less time per side usually.  

the easy way to create a compilation is to digitise everything.
an exception is paul simon's negotiations and love songs.
engineer roy halee hand assembled all the cuts in analogue, the result is a superb sounding set that often sounds better than the original presentations.
other great sounding complilations include the eagles - their greatest hits 1971-1975, steely dan greatest hits.
nb. only in their original releases, newer re-issues generally do not sound as good.