[Arkan]

Thats a very tough question that can't really be answered. You need to find groups or individuals who changed the way music was/is played.

Elvis Presley - The "King" of rock-n-roll

Beatles - I only recently started liking them...but regardless, they were ahead of their time.

Queen/Led Zeppelin/Rush - very innovative bands of the 70's.

Metallica & Van Halen - Biggest bands of the 80's

Nirvana & Tool - These bands ruled the 90's and developed a new breed of rockers.

Present - Not too sure yet....this decade just started but 3 Doors Down and Godsmack are making their charge.

As for Rap, the Sugar Hill Gang started it all....followed by Run DMC. These guys were the pioneers of rap....everyone came after.

80's Freestyle - TKA kicked off the Freestyle frenzy in NY with many artists gaining notoriety because of them.

regardless if you like them of not, these groups had a big influence in their respective musical genres.