I'm not going to read through 34 pages of comments, so I'm sorry if any of this was said before. But...
System of a Down's entire library should be available, if you ask me. I know, I know. We should include other bands that aren't yet available before thinking about bands that have a few songs already. I'm still waiting on a few bands myself. (The Used, My Chemical Romance, Hikki, 311, Pink Floyd, Queen...)
However, the difficulty of all the songs as a whole would be pretty wide-spread, from Solid (as a band, at the lowest) to Impossible, which would appeal to a vast majority of players. Serj has a pretty... unique vocal style, to say the least. Insane guitar, insane drums, insane bass. Fun for all, right?
While we're at it, any and all of Serj's stand-alone songs would be great, especially with his new album coming out late this year (so I've heard). We could also use the rest of Scars on Broadway, since They Say sucked. All of System's covers and collaberations (Will They Die 4 You?, Marmalade, Metro, Shame, and Snowblind) would be great. All their live work. Serj's original band's work, Daron's original band's work, Shavo's band AcHoZeN's work... Basically, anything and everything to do with System of a Down and the band members before and after forming the greatest band ever.
If only I was joking. I'm going to need a back-up hard drive.