[CausticGnostic]

The question's rather open-ended. Do you mean recent musicians/bands, or musicians/bands from all of history?
If the former, just since the 60s and only in rock, my top three are:
1) The Beatles
2) Led Zeppelin
3) Pink Floyd
If the latter, though, and favoring anonymous groups of musicians that collectively altered our understanding of music and going in historical order, then:
1) the troubadours of Provencal (who popularized love songs, which are still with us);
2) the European classical composers (who broke orchestral music away from purely liturgical uses)
3) African-American blues and gospel singers (who provided the roots of R&B, swing, jazz, and rock music).

[Mademoiselle de Lioncourt]

1. Elvis Presley

2. John Lennon

3. Slash <--- The guitarist from Guns and Roses. xP