It's a close one between the well-known National Lampoon's Vacation and this much-lesser-known gem, as to which was the best Chevy Chase film...
http://upload.wikimedia.org/wikipedi...vie_poster.jpg
The poster makes it look really, really old.
And it kind of is -- released in 1980.
However, it holds up very well, and most of the movie could easily take place today.