Okay, this one is totally subjective, no criteria whatsover, it just depends on what makes you laugh.
Personally, I can't recall a big long list of really funny movies. Many where kinda humorous, but how many films did I actually laugh out loud while watching? Not many. I'm not the kind of guy who grabs his gut and doubles over laughing when Lucy sticks a bunch of chocolates in her mouth.
So my contenders for funniest movies are these:
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Ted (2012 Seth MacFarlane film with Mark Wahlberg): absolutely fucking hilarous. One of the few movies in memory that had moments when I'm in tears from laughing
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40 Year Old Virgin: Hilarious, I could watch this movie over and over and over.
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Caddyshack: Very funny, though some long boring sections. But some of the funny stuff in Caddyshack was legendary.
After those two I'm pretty much drawing a blank. Yeah, some early Woody Allen stuff (Take the Money and Run), some Mel Brooks stuff (Blazing Saddles) was very funny, but otherwise, I'm drawing a blank. I'm sure I'm missing a lot of great stuff...


and for you very old guys 5) Mister Roberts