I would not say that I'm a good Guitar Hero player, but I like the game.
In the past few weeks, I've taken a keen interest in beating Slayer's "Raining Blood". It's the one song that I've gone into practice and have worked on techniques to make it through various parts of the song. I consider this to be one of the hardest songs in Guitar Hero III.
Roughly 15 minutes ago I beat it.
Exhibit A, Exhibit B, Exhibit C
Here is a glimpse of the torture that is "Raining Blood" (no, this is not me playing).
Update: I love this gem from the song's Wikipedia entry.
"Raining Blood" is a playable song in the video game Guitar Hero III: Legends of Rock, where it is renowned for being one of the hardest songs in the Career Mode setlist, mainly due to one section where you must execute pull-offs at a rate of ~14 notes per second.