The Chicxulub crater hypothesis is, still, a hypothesis (not a theory. Pet peeve), but it is really well substantiated and modeled and that crater absolutely did some fucking damage. Odds are it was multiple things, and the extinction took a few years at least, if not a few thousand. The iridium layer is a biggie.

http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event

EDIT: The main objection to the Chicxulub crater hypothesis is

Gerta Keller, however, suggests that the Chicxulub impact occurred approximately 300,000 years before the K–Pg boundary.[11] This dating is based on evidence collected in Northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs approximately 10 m (33 ft) below the K–Pg boundary. This chronostratigraphic sequence of rock is thought to represent 300,000 years. This finding supports the theory that one or many impacts were contributory, but not causal, to the K–Pg boundary mass extinction.[11] However, many scientists reject Keller's analysis, some arguing that the 10-m (33-ft) layer on top of the impact spherules should be attributed to tsunami activity resulting from impact. Few researchers support Keller's dating of the impact crater.[15]