Some people point to 'Mogwai Young Team', others back to Slint. However, I believe (and have stated previously) that the true father of the post rock genre is EDGAR ALLEN POE. Check out his description of Usher (not that Usher) playing frantically on his guitar:

“I listened, as if in a dream, to the wild improvisations of his speaking guitar.”

“His long improvised dirges will ring for ever in my ears.”

“It was, perhaps, the narrow limits to which he thus confined himself upon the guitar, which gave birth, in great measure, to the fantastic character of the performances. But the fervid facility of his impromptus could not be so accounted for.”