The history of rap music is in many ways a history of false starts and missed opportunities. At once popular music's most cannibalistic genre and an outlet for limitless creativity, rap is littered with brilliant acts that ran out of steam, great ideas that went nowhere, and subgenres that stubbornly refused to evolve. Yet, through it all, rap has persisted, evolving from the soundtrack to a nation of break-dancers and graffiti artists to a huge international industry with limitless commercial potential. Still, listening to Afrika Bambaataa's revolutionary "Planet Rock" on the four-disc, 56-song Tommy Boy's Greatest Beats box set, it's difficult not to conclude that rap has come nowhere near fulfilling the boundless promise inherent in that song. With "Planet Rock" and his legendary collaborations with John Lydon and James Brown, Bambaataa seemed to point the way toward a truly revolutionary form of music, a style that would make up the rules as it went along and bridge even the most formidable societal boundaries. When James Brown joyously yelps "punk-rocka, new-wave-a" on "Unity," it's a ridiculous moment, sure, but there's sweetness and idealism in it, as well. Enormously rewarding but wildly uneven, Tommy Boy's Greatest Beats is full of those moments, when rap music seemed on the verge of something transcendent. Unlike Def Jam, which boasted both a recognizable sound and a number of great acts, Tommy Boy has employed only two consistently stellar performers: Bambaataa and De La Soul. But what Tommy Boy lacks in superstars it has made up in solid acts such as Digital Underground and Coolio, both of whom have proved adept at turning out summer-defining anthems. Tommy Boy has also boasted such similarly radio-ready acts as Flavor Unit stalwarts Queen Latifah and Naughty By Nature, not to mention guilty pleasures K7 and House Of Pain, whose entire worthwhile output (two songs apiece) is helpfully collected in its entirety on Greatest Beats. Unfortunately, the label also featured a slew of anemic dance-pop acts—Force MDs, Club Nouveau, TKA, and more—who are sadly over-represented on the box set. Paris, possibly the most underrated artist in rap history, is allotted only one song, while the justifiably forgotten Club Nouveau gets three, only two tracks fewer than De La Soul. Still, despite questionable song selection, Tommy Boy's Greatest Beats is all but essential, a rousing look at hip hop's shaky evolution that, as a special bonus, also features a nifty remix disc featuring classics reworked by such young techno turks as Jason Nevins and Deejay Punk-Roc.