I Am Legend - Not so much a film about the end of the world as it is about the end of humanity. Still an entertaining tale of one mans survival in New York city after the entire human race is wiped out by a virus. Will Smith does an excellent job in the film, carrying about 3/4 of it by himself as the only human character for a majority of the movie. I would also recommend the other two films based on the book "I Am Legend", The Last Man on Earth and The Omega Man.
Armageddon / Deep Impact - These two films have similar plots and were released roughly around the same time. An asteroid / comet is heading to earth and unless its destroyed everything on the planet is dead.
Personally I prefer Armageddon since its a more of a fast paced film. Featuring a great cast including Bruce Willis , Ben Affleck and the late Michael Clarke Duncan. Michael Bay directed the movie, so you know there will be tons of action and explosions happening.
Deep Impact focused more on the story and had less of an emphasis on action. It had a great cast including Robert Duvall, Elijah Wood and Tea Leoni. You cant really go wrong with either film.
These are just my favorites , there are a ton of other movies out there with different themes. You have aliens in Mars Attacks and Independence Day, Zombies in Dawn of the Dead and The earth itself in movies like The Core. Surprising we like to make so many movies about our world ending, isn't it?