Guitar hero 2, because of the fact that the songs of GH2 are actually challenging, unlike the others.
In Guitar Hero 3, I found very few songs which were actually "hard".
These ones were:
The number of the Beast - Iron Maiden.
Raining Blood - Slayer
Through the Fire and Flames - Dragonforce.
Buuut, in GH2, I found some really challenging songs, such as "The Beast and the Harlot" by Avenged Sevenfold, where it was pretty challenging because of the fact that I use the joystick to play, instead of the guitar.