I do not listen to radio, so I do not know which songs were released in 2008. I think I had All I Need on my 2007 list and In Rainbows would be at #1 for this year, if I were to cheat just a little. But, I had it in last year's top 5 (last year was that good).
By the way, if I had cheated a little bit, the other possible # 1 would be the 2008 re-release of K'naan's The Dusty Foot Philosopher, in the Top 10 for 2006 and the most listened to album on my iPod the past 2 years. But, i did not buy the American re-release.
But, if you are looking for the best rap album of the year, it should be bought. Expect his next album Troubadour to be in next year's top 10, unless he pulls a Jenny Lewis.