kataklysm has riffs crushing shit like tanks. This song is one of the reasons I got a seven string (eventhough they don´t use sevens but B standard tuning).
I like pretty much everything posted in this thread but I feel strongly that any "riff's that flatten buildings" posted that aren't Crowbar riffs fall pretty short.
Thou - Ode to Physical Pain. The riff is at 3:00. ....ing CRUSHING. Happy to say that I got to see them live in a shot out basement. They played all new songs from Heathen and basically deafened EVERYONE. The new album is absolutely superb. It's up for "name your price" on Bandcamp right now, so do the guys a favor and download the album and throw them a few bucks or order some merch or vinyl from them. They deserve it.