According to Robert Hingley, singer and lone remaining founding member of The Toasters, the answer is — surprise! — The Toasters. Formed in 1981, the band outlasted both the 2 Tone (English Beat, Madness) and Third Wave (Mighty Mighty Bosstones) eras of ska and has never gone more than five years without releasing a studio album. Along the way, they have toured continuously and racked up 35 former members.
One mystery solved, several more started. Because if we accept The Toasters as our Methuselahs of ska, doesn't it make you wonder about rap? R&B? Swedish death-metal?
Refer to the following list to find your favorite genre and its respective longest-continuously-running act. (Disclaimer: Only Billboard 100 and other commercially viable acts were considered. Your great-uncle's Sun City polka combo doesn't count.)
Heavy metal: It's gotta be Judas Priest, right? The leather-clad "Breaking the Law" rockers started playing blues joints in Birmingham, England, way back in 1969 and released an LP (Nostradamus) as recently as 2008. But their claim to fame as heavy metal's elder statesmen holds up only if you count the four years when they had no lead singer (1992-96) as "active." Our buddy Rob Halford might have something to say about that.
Rap (solo artist): Just 20 years old when he signed with Mercury Records and released "Christmas Rappin'" in 1979, Harlem hip-hop pioneer Kurtis Blow (né Walker) is the undisputed Neil Armstrong of flow. Officially still active, Blow now rhymes for Jesus as an ordained minister.
Rap (group): How did three nice Jewish boys from Brooklyn become rap's most enduring institution? In 1979, 14-year-old Mike Diamond (a.k.a. Mike D) started a hardcore punk group called The Young Aborigines, which later re-branded itself the Beastie Boys with the addition of bassist Adam ("MCA") Yauch and guitarist Adam ("Ad Rock") Horowitz. The trio got solid returns from the 1983 release of a hip-hop 12-inch called "Cooky Puss," and have been dropping mad hits like Rod Carew ever since.
Rock: The Stones. Duh.
R&B: Launched when a goodly portion of baby boomers were still in diapers, The Drifters have been doo-wopping hits ("Under the Boardwalk," "Up on the Roof") since 1953. Granted, this is not a self-governing band by any measure. The musicians are hired and fired Menudo-style by the Treadway family, which has owned The Drifters copyright since the '50s.
Electronic/synth: According to their bio, Kraftwerk has been making inscrutable German synth art since 1970. Can that be right? Did they even have electricity in 1970?
Country: The Oak Ridge Boys have existed, in one iteration or another, since 1943 — the same year that current lead singer Duane Allen was born. It's a fairly astounding feat of brand preservation, when you stop to think about it. They outlasted the Cold War and most likely will outlast the Cold War Kids.
Punk: This is the hardest genre to quantify, because — let's face it — most punk rockers are degenerates who have no reliable memory of where or when they first played. And how many of them were ever commercially viable? So we'll go with Social Distortion (1979-present), which stayed officially active even when Mike Ness was off doing his "solo stuff" — i.e. rehab stints, drug busts, solo albums.
