I love the start of unforgiven 2 by Metallica - not really inspiring but the whole song is just such an epic and it sets the tone. And obviously master of puppets intro is superb, just awesome, not to mention fade to black, breadfan and am I evil?...

Metallica do have some awesome intro's, but outside of them -

Iron Maiden's fear of the dark is a good 'un, as well as the prisoner, also by them - I am a free man!

Blind Guardian's - And then there was silence is very good as well.

Too damn hard to pick just one ;p