I thought this thread would be about if i were an evil villain, in which case i would develop a lethal virus that only works on humans and is transmitted by any means conceivable.
But since i'm being given the position of a god and i wish to kill ALL life, i'd send the earth slowly but steadily on a collision course with the sun. I'd be a spectacle worthy of a god, and something to keep my mood positive, as i realize i have to start from scratch.
EDIT:
my favs so far (besides my own) are Zombies (infinite source, since we all die at some point) and SimCity-style eraser of meteorites, volcanoes, aliens, giant robots, giant monsters, floods, etc. (pretty much all i did, i found the actual game to be pretty boring).
And in response to the seemingly popular use of nuclear weapons for this task, i offer this: