I tried to make the poll unbiased.
I realize that we have had a couple of their songs already, but I wouldn't mind more. Personally, I have never liked "Should I Stay or Should I Go".
Obviously, most songs by the Clash would not be difficult, and it would really come down to whether you like the song or not, rather then the charts. I wouldn't mind "London Calling" as a single song. I imagine the difficulty would fall between Complete Control (my favorite Clash song on Rock Band so far), and Train in Vain.
Also, I think that the London Calling album would be a good choice for a full album download. Here is my reasoning; all three (four?) albums we have had to date have been from bands who were extremely influential to their genre. Judas Priest to Heavy Metal, The Cars to New Wave, The Pixies to Alternative, and Boston to rock in general. I'm not really a fan of punk at all, but I think London Calling is a very influential punk album, and would be a good choice for the genre.