Graspop 2004 was great. My favorites from that festival were Iced Earth, Anathema, Evergrey, Fear Factory, Testament, Anthrax. Some bands like My Dying Bride, Motorhead and Killswitch Engage put on a good show, but their music wasn't too much to my liking.
Other bands on the bill: Morbid Angel, Judas Priest, Alice Cooper, Saxon, Slipknot, Shadows Fall, Hypocrisy, Dimmu Borgir, Cradle Of Filth, Suffocation, Dillinger Escape Plan, Epica, Children Of Bodom, Therion, Soulfly, Queensryche, Death Angel, Dying Fetus, Malevolant Creation, After Forever and also a few more...
Great lineup, since it has a bit for everyone.