The original 5-manning of the Deathtalon Wyrmguards in Blackwing Lair - complete with epic loot.
Within 12 hours of this video's release, Blizzard doubled the run speed of the Wyrmguards to make such a stunt impossible. But, the crafty individual can surely find a way to farm other BWL mobs in similar fashion.
NOTE: This video occured in patch 1.7 (Zul Gurub Patch).
75% of this video is in the mob pull itself (especially after we figured out how to just get one). The rest is just running around using just about everything available in-game to increase run-speed, and praying we get a fire, frost, or shadow Wyrmguard.
Another note: such high criticals are possible because certain mobs in BWL are extremely weak to one school of magic, which is what makes this run possible (Being able to do enough damage before we get bored and give up)..