50's - Elvis and Little Richard (Both brought rock to the front of the music scene)

60's - Beatles (Obviously)

70's - Marvin Gaye, Stevie Wonder, Queen, David Bowie, Elton John, and Led Zeppelin (All very successful, legendary, and influential acts who maintained success throughout the whole decade)

80's - Michael Jackson, Prince, and Madonna (The holy trinity of pop music)

90's - Mariah Carey, R. Kelly, and Janet Jackson (All 3 contributed heavily to mega-transition R&B went through in the 90's, the genre which was arguably the biggest that decade)

2000's - Beyonce and Usher (Hit after hit they've kept going and are faces recognized around the world)

[Edited 3/4/13 14:27pm]

it is hard to say...there were a lot of artists that reign in all decades:

1950s=Little Richard, Fats Domonio,Elvis Presley, Jerry Lewis,The Platters,Chuck Berry,Bo Diddley

1960s=The Rolling Stones, The Who, THe Beatles,The Beach Boys, The MOTOWN SOUND(The Supremes, THe Temptations, The Four Tops, Mary Wells, SMokey Robinson and THe Miracles, Marvin Gaye, and etc.), Dionne Warwick,Otis Redding

1970s=Earth, Wind and Fire, jackson browne, james taylor, carol king, Parliament/Funkadelic, Barry White, Curtis Mayfield, The Ohio Players, The Bee-Gees, Roberta Flack, The O'Jays, Aerosmith, Kiss, Steely Dan, Stevie Wonder, The Grateful Dead, The Doobie Brothers,Elton John,

1980s=Prince, Michael Jackson, Bruce Springsteen, Madonna, Whitney Houston, Janet Jackson, tina Turner,Culture Club, George Michael, New Edition,Cyndi Lauper,Luther Vandross

this is a hard one because you had all different genres and these artists(or artists not mention), made some great music..but if I had to narrow it down

1950s-Elvis/Little Richard/Ruth Brown/Ray Charles

1960s-The Beatles/The Supremes/The Rolling stones/The Mamas and The Papas/The Beach Boys/otis Redding/The Temptations/Dionne Warwick

1970s-Stevie Wonder/Carol King/Earth Wind and Fire/Elton John

1980s-Prince/Michael Jackson/Whitney Houston/madonna/Janet Jackson