"Master Heartache" by Sir Lord Baltimore. The lead riff that starts playing behind the singer right when he goes "I know there's evil/ in the ways of the wicked (whatever he says)" It's the best. Every time it happens in that song it's the best thing ever.
I also agree about the transition from acoustic intro into the full band on Tractor Rape Chain.
I also think the first 4 seconds of palm-muted build up in "The Lung" by Dinosaur Jr.