You know, I'm really starting to realize I'm becoming a perversely-obsessed 'Monster' fanboy.
But, again, an honest answer -
'Grain' (Monster opening)
'For the love of life' (by David Sylvian, ending)
The ending, for me, is a huge treat - this is because David Sylvian, even without having anything to do with anime, has been my favorite vocalist/lyricist for quite a while. So, I think it's easy to justify why.

The Birds are using humanity in order to throw something terrifying at this green pig. And then what happens to us all later, that’s simply not important to them…