As is true of even the best country artists, Reba McEntire's albums tend to consist of two or three killer tracks plus a lot of pleasant filler. That makes greatest-hits albums the way to go for both quality and economy. With McEntire, that also gives you a sample of each phase of her career: * The Best of Reba McEntire (Mercury) collects her most hard-core country laments from the early '80s-Reba's twang has never sounded so elastic and / resilient. * Reba McEntire's Greatest Hits (MCA) documents her late-'80s rise to superstardom, emphasizing busted-romance ballads like ''He Broke Your Memory Last Night'' and ''Whoever's in New England.'' * Greatest Hits Volume Two (MCA), her latest release, features ''Does He Love You,'' the duet with Linda Davis that had 'em whooping it up at the CMA Awards, as well as the passionate throatiness of pop-country smashes ''For My Broken Heart'' and ''Rumor Has It.'' All three of these discs are Reba-rrific and deserve an A.