It's a hard question. From the metal side of things it is, anyway. Most decades, particularly the '80s, '90s and '00s have seen their own mainstream annoying trend. '80s had awful hair metal, yet that decade was the decade of the major Thrash metal movement, bringing in bands like Slayer, Megadeth, Anthrax, Metallica etc. The '90s had the moving in of Grunge and Nu-Metal (or Rap metal, to some) yet was the decade of excellent bands like Pantera (CFH and afterwards), Carcass, Machine Head and Down.
So far though the '00s have been the absolute worst, particularly with the moving in of post hardcore, generic emo and the current metalcore thing. It all sounds so samey, and to be perfectly honest I haven't seen too many new, good bands that started in this decade that won't be forgotten within the next few years and will die out.
The '90s win it for me though, generally Grunge and Nu-Metal was somewhat likable. Plus bands like Pantera are, despite now being non-existant, still one of the biggest bands ever, along with Carcass.