ALL RAPPED UP AND READY TO GO

Like any top-selling genre, hip-hop is widely represented on compilation albums. But only a few rappers have greatest-hits retrospectives of their own: Jamming lead guitar into the mix was just one of Run-D.M.C.'s innovations; the devastating trio gave rap the momentum to reach the rock audience. Together Forever: Run-D.M.C. Greatest Hits 1983-1991 (Profile) carefully chronicles the verbal gymnasts' brilliant career, including ''Walk This Way,'' their pigeonhole-busting 1986 collaboration with Aerosmith. A... Hip-hop's best-selling female group doesn't get a lot of hard-core respect, and the inconsistency of A Blitz of Salt-N-Pepa Hits: The Hits Remixed (Next Plateau) explains why. Tough rhymes like ''I'll Take Your Man'' sizzle with conviction, softer-hitting dance tracks don't. C+... As a svelte synopsis of the tubby trio's first three records, The Best Part of the Fat Boys (Sutra) is enormous fun, a smorgasbord of comical rhymes. B+... Despite the grossly misleading title, Kool Moe Dee: The Greatest Hits (Sugar Hill) is a valuable document of rap's formative years. It's full of entertaining party rhymes and now-rare singles by Moe Dee's group the Treacherous Three that predate his prosperous solo career. B-... Lightweight content dressed up with fancy production, Whodini: Greatest Hits (Jive/RCA) showcases this Brooklyn group's easygoing commercial style. B-