I've never tried it before and I'm going to Montreal next year, so where should I go for the absolute best poutine?
This is always a question of personal taste. I personally like some out of the way places like La Belle Province on St-Jacques street. But if you're planning on being downtown (and I assume you are), the most popular with the locals is La Banquise Resto. Generally considered the best poutinerie in Montreal. Expect to stand in a line though, particularly at night after everyone's had a few drinks and start to get the alcohol munchies.
Au Pied de Cochon.... Bar none the best poutine in the world. Don't let anybody tell you otherwise. There is nothing on this earth better tasting than the pouting form the Pied de Cochon. Bon apėtit