1. SEGA going third party
2. FFXIII on Xbox360 (Sony is doomed!!)
3. Nintendo wii mote revealed annex Nintendo is doomed!!!
I would put Nintendo Wiimote reveal at 2, bump FFXIII betrayalton to 3. Though surely nothing will ever compare with SEGA leaving the hw business, that prompted threads for weeks and launched meltdowns for years after the announcement (only saw this through someones GAF archive, ah ancient history GAF). The only thing that might get close would be any of the current manufacturers going third party or going bust. And because this is a young industry and hasn't happened a huge amount yet, any of the great games creators or company founders / bosses passing away would be pretty big.
PS3 price announcement would be in a top ten surely and DQ to DS too, that was pretty insane.