For a "blockbuster title" ala Halo or God of War, I guess a flashy E3 presentation is the best option. However, social media and word of mouth have been used very effectively to market smaller indie titles, and in some ways I prefer that method because it tends to foster a stronger connection to the game/developer and create a bit of a community around a game before it's even released.
Yea I agree it really depends on the type of game you're trying to get out there. For larger AAA titles or games with mass market appeal you got to the go the way of flashy press conferences n shit. Otherwise, for smaller titles or even great niches titles like Demon Souls and (at the time) Castlevania: SotN that got big off of word of mouth I like that method too.
I don't know too many games that utilized a viral marketing campaign, sounds like something that'd be pretty damn cool when I think about it.