I started with the list from American Film Institutes Top 100 - http://www.afi.com/100years/movies10.aspx (although there are way too many R-rated movies on there so it really limits what I would recommend)


It's a Wonderful Life
Mr Smith Goes to Washington
Twelve Angry Men (I'm a Jimmy Stewart fan)
Bridge on the River Kwai
Gone with the Wind
To Kill a Mockingbird

Alistair MacLean movies:
Guns of Navarone
Force 10 from Navarone
Where Eagles Dare
Ice Station Zebra

Comedies:
Airplane 1 & 2
Stuff from Zucker
Monty Python
Pink Panther movies with Peter Sellers