Oh, fucking shit. The best of the best, huh? Yeah, this is VERY hard to choose right here. All the songs are top-notch.
Aye, since one song MUST go, then it's going to have to be
Gateways of Bereavement, and that is indeed one amazing song which I do favour above many other Katatonia songs (new and old.) It's not like I necessarily like it the least here, but it's the song I'd probably listen to the least out of the bunch.
Edit: WAIT...haha, that was close. I JUST forgot about Deadhouse. Yeah, I love it to death but it isn't my #1 or even #2 choice off DO, while Gateways of Bereavement happens to be one of the greatest black metal songs I have ever heard - eh, black/doom/funeral doom (yeah, it does remind me alot of funeral doom hehe.)
Last minute calls, and the vote goes to
Deadhouse.