Most of the greats have been listed but would "The Day After Tomorrow" count? Not so much a post-apocalyptic movie but falls under the category of an end of the world type scenario.
I would say:
28 days/weeks later
George A. Romero's zombie flicks (night/dawn/day/land of the dead) With a special nod to the original "Night". They're coming to get you Barbra....they're coming to get you....
Planet of the apes (old school version with the great Mr. Heston, NOT the crappy Tim Burton remake...it's too bad about that too. I would like to have seen updates to all of the "Apes" films.) plus the sequels.
Mad Max (of course) plus the sequels.
Speaking of I am legend, we must not forget "The Omega Man" with Charlton Heston. It was the second film version of that book. Personally I must say that I actually really like the old Vincent Price version (Last Man on Earth) quite a bit....not sure why but it has the most "desolate" feel to it. It sort of makes you feel like you are there.