Very tough but in terms of mainstream rock and pop it is this current decade.I have to say there is not really an era that produced the worse music.
I am a huge music fan and love 90's grunge, 80's hair metal, 90's rap, 70's classic rock. However, in the 00's there seems to be a ton of non-mainstream artist that are unique and amazing. No larger than life, Aerosmith, Led Zep, Metallica, Pearl Jam type bands but a ton of good ones.
Turn on Octane formerly squiz on XM radio and you will hear a ton of new awesome heavy tunes if that is what you are into. Five Finger Deathpunch,All that remains,Godsmack, Sevendust, System of A Down,P.O.D., Mudvayne, all seem unique to their generation. The 00's brought out the heavy female lead singer rock band stronger than ever like Lacuna Coil, Evanescense, OTEP, etc. Tough call but I have to still say 00's although there isn't really a bad decade in my opinion.