Chevy Chase for Christmas Vacation. I haven't seen the other vacation movies, but the guy just can't act to save his life.
That's an odd one - especially since it's a screwball comedy where acting ability isn't usually scrutinized.
A big factor here is you haven't seen any of the others and most people (myself included) feel the first movie of the series is the best. Since Chevy has played the same character multiple times he's become expressly associated as that character.
(Personally, I feel
'Christmas' breaks with the others as it's not a travel movie like the other 3. For me,
Christmas Vacation is a holiday movie while the others are "road trip" movies. Not that it's lesser, just a different premise from the rest.)
Clark Griswold is an entirely tongue-in-cheek character to begin with - he's kind of like the baser instincts of everyone's dad manifest in one person - he's not a great guy: he's conniving, sarcastic, not always honest, often self-centered, he's concerned about impressing others, he's vain, let's his temper get the best of him, does not always set a good example to this children, is lustful (coming close to committing adultery but never crossing that line), likes to gamble, becomes reckless & irresponsible when under stress, and is easily frustrated while always trying to maintain an air of coolness and a façade that he's in control.
From things I've heard about Chevy Chase - it seems like "Clark" isn't all that far off from his own character.
For fans of the series, it would be hard to imagine anyone else in the role - strangely enough, he and Beverly D'Angelo were the only true consistencies throughout the 4 films (as their children were played by different actors in each one).