Pantera's album cover for "The Great Southern Trendkill".
 
Pantera was a highly influential groove/thrash metal band from Arlington, Texas. They have been cited as one of the biggest influences on modern heavy metal, and are usually named as influence by nu-metal bands.
Pantera formed in 1981 and started as a glam metal band, but on their 1990 release Cowboys from Hell, changed their sound into a much more aggressive tone. It was new at the time, and Pantera picked up on the fame that came with it. They released nine studio albums before they split up in 2003 due to internal issues.
Their entire album The Great Southern Trendkill is available as downloadable content for Rock Band, with the exception of the song "Suicide Note, Pt. 1" (most likely due to the fact that it lacks playable drums and is mostly atmospheric).
All of the songs available are usually fairly difficult on guitar and VERY difficult on drums, but most of them are easy to sing (with the exception of Suicide Note Pt. II, which is 5 stars on vocals).