Soundwave on 14 September 2012
Dragon Quest IX and Resident Evil exclusivity on GameCube was probably bigger.
Final Fantasy breaking from Sony exclusivity was pretty huge too, even the Wii U will probably benefit from that this gen (so thanks Microsoft).
Resident Evil + Capcom 5 was the big one for me. But that happened like the day of Sept.11th, 2001, so it was like incredibly weird, all this stuff was happening and then there's this major Nintendo/Capcom announcement. Surreal.
