This week, Last.fm revealed the first part of its annual Best of 2010 feature, a countdown of the year’s most-played music that’s entirely based on tracks scrobbled by Last.fm users.

Over the next three weeks expect to be surprised, delighted and horrified in equal measure by the names in the top 40, because from pop divas to rock gods, Best of 2010 has it covered. Last year, Lady Gaga topped the chart: who will it be this time?

The Last.fm Best of 2010 doesn’t count re-releases, live albums, compilations, or greatest hits collections; just new albums released this year. That means we’re guaranteed a fresh face at the top of this year’s chart.

Will it be an icon like Christina Aguilera, Gorillaz or Arcade Fire? Or will it be a newcomer, like Mumford & Sons, Ke$ha or Beach House? You’ll find out over the coming weeks who’s really on top.

As with last year, you’ll be able to dig in your own personal “best of” soon, with a raft of fun features released in the final week of the countdown.

This week’s list reveals #40 – #21 on the countdown. Next Wednesday, Dec. 8, Last.fm will reveal #20 – #11, and then on Wednesday, Dec. 15, we will finally see the list of Last.fm’s ten most scrobbled artists of the year. So stay tuned!

Find out who’s at #40 – #21 right now…