Adam at 2011-05-16 17:10:17:
The book "The A List" from 2002 or so, edited by Jay Carr, is also a very solid list that includes some more off the beaten path suggestions, and includes foreign film, unlike the AFI lists (for obvious reasons).
Also, the Criterion Collection is another great set to work through. They cover a wide range of movies, ranging from true classics to unheralded oddities, but everything they put out is worth a viewing.