I don't think I would be able to pick a single funniest movie of all time, but there are a few on the shortlist.
Horrible Bosses
The Hangover
In my opinion, these two were so well done, they should not have had sequels. I know, I know, boatloads of cash, but still...
Groundhog Day
Bad Words
Spies Like Us
Stripes
The Lego Movie
Obviously not a comedic masterpiece, but I just happened to stumble across "Down Periscope" the other day on an older hard drive, so I gave it a re-watch.
Still quite funny, held up rather well over the years.