Every great rap group has one MC who is-- possibly unfairly-- perceived to be slightly lesser than the other. DMC. Parrish Smith. Malice. Pimp C, at least up until he died. Big Boi's been on that list ever since André Benjamin started rocking pith helmets and neckerchiefs. Big Boi's not underrated, exactly; everyone who knows rap knows he's a great rapper. It's more that he's taken for granted. Virtually every OutKast review of the past decade and a half has posited Big Boi as the earthy, street-level anchor to André's spaced-out visionary, the guy responsible for securing the group's cred when André was trying to invent new colors. Expect Sir Lucious Left Foot to change those conversations. We haven't heard a major-label rap album this inventive, bizarre, joyous, and masterful in a long time, and it's almost impossible to imagine André putting out a solo album this strong anytime soon.

At this point, Big Boi has every right to indulge in the bitter-old-man invective that's tempted so many other rappers of his generation. Even though he's half of one of the most successful groups ever, Big Boi has had to go through years of release-date delays and label drama (some of the topical lyrics here sound like they were written years ago), until he finally left longtime home Jive just so he could release a damn solo album already. Label machinations kept André's voice from even appearing on Sir Lucious Left Foot-- heartbreaking when you think about André's jaw-dropping display on the early advance single "Royal Flush". But instead of letting these setbacks infect his music, Big Boi's made an album that explodes with ideas at every turn, that glides and twitches and mutates with delirious urgency.

Musically, the album drips with 1980s synth-funk signifiers. The keyboards glimmer as they roam, and talkboxes mutter and blurt. But these tracks aren't the stoned miasmas that someone like Dâm-Funk cranks out. Instead, they're itchy and fleet-footed. New melodic elements flit in and out of tracks just as you start to notice them, and there's a lot going on at any given moment. Consider, for example, album closer "Back Up Plan". The track, from old comrades Organized Noize, finds room for cheerleader chants, disembodied grunts, a weird little synth whistle, processed funk guitar, orchestra hits, frantic scratching, a lowdown wobbling bassline, and probably some other stuff that I'm missing-- and this is one of the most laid-back songs on the entire album.

Every once in a while, we'll get a nod toward some current trend, but these aren't market capitulations; they're more opportunities to play with what the kids are doing now. "Follow Us", for example, has a generic rock-dude chorus from Vonnegutt, and virtually any other rapper would've built a hackneyed half-rock song out of a chorus like that, but producer Salaam Remi instead piles bubbly synthetic melodies all over each other. And the robo-voices on "Shutterbugg" aren't airy Auto-Tune; they're more of a deep rumble that you can feel in your gut. "Tangerine" somehow simultaneously sounds like strip-club ass-shake material and Funkadelic covering Morricone. Looking at the production credits, it's surprising to see names like Scott Storch and Lil Jon-- hitmakers who don't really make hits anymore, and who haven't been all that interesting in a while. So Big Boi, then, is someone who encourages his collaborators' furthest-out ideas, and who knows what to do with those ideas when he gets them.

As a rapper, Big Boi is something else. He just does so many things with his voice and cadence, letting his words fall over the snares one moment and fighting upstream against the beat the next. He never falls into any particular pattern of delivery, instead using his flow to knock beats back and forth with relish. Sixteen years after the first OutKast album, he's still coming up with dizzy combinations of words: "My recitals are vital and maybe needed for survival," "The slickness that get your chick hit quick," "Stay sharp as broken glass, get busted on a smash/ When your ass cross paths with this half of the 'Kast." Even if he were saying nothing, the tumble of his words is a thing to behold.

But then, he's never saying nothing. Sir Lucious Left Foot is an album blissfully free of both old-man hectoring and drug-rap nihilism. A few times Big Boi brings up the idea that it's really not too smart to rap about selling crack all the time, but he doesn't dwell on it, and he confines most of his skepticism to awesomely worded asides ("Snow? That's for toboggans.") He's hard enough to tell you to get the South's dick up out your motherfucking mouth and draw blood with the command, but he's also clever enough to slide away from threats just as quickly. He spends a large chunk of the album talking about sex, sounding like a fired-up 11-year-old goofing off in the back of some sort of prodigy-level English class.

There's a lot more to like about Sir Lucious Left Foot. Some of the skits are actually pretty funny. The guests-- who range from masterful fast-rap newcomer Yelawolf to a stirringly gritty Jamie Foxx-- all turn in top-shelf performances. Old Dungeon Family associates Big Rube and Khujo Goodie make feel-good cameos that actually contribute to their songs. But the real story is the rap veteran who's done everything he could possibly do in the genre but who still finds new ways to have fun with it. Last year's best rap album came from Raekwon, another wily old vet who hit a serious late-career stride. But Raekwon did it by inhabiting his older styles, making a record that could've conceivably come out in 1996. On Sir Lucious Left Foot, Big Boi does something even more difficult: He gives us a great album that sounds nothing like any of the great albums he's already given us. From where I'm sitting, that's an even greater achievement.