Cassablanca, Bell, Book and Candle, Most of Alfred Hitchcock's work, On the Waterfront, The Longest Day, King Kong, In Harms Way, They Were Expendable, Wake Island, any Cagney or Edward G. Robinson movie, to mention only a few ruined by colorization.
King Kong, where he looked liek he'd had too much coffee.
Get a bunch of soldiers together with some suds and popcorn, and listen to them MST3K the people running from Kong.