Ghostbusters is easily one of, if not in fact the funniest movie ever made. (It's the SRFIAs #1 Comedy of All Time, and is in the top five Greatest Films of All Time.) There are so many comedic elements that came together to make this one perfect. Bill Murray and Dan Aykroyd alone could have been the only actors and it would have still been excellent. Add Harold Ramis and Aykroyd as writers? Well there are so many quotable one-liners in this movie it's ridiculous. And then put a slice of sassy black man (Ernie Hudson) in the mix, and a sprinkle of Rick Moranis and you've got legitimate comedy gold. (Trivia note: they were originally wanted to give the role of Luis Tully to John Candy!)
One of the greatest aspects of Ghostbusters is that it's not just a great comedy but also has a element of actual scary movie to it. Real ghosts doing scary things. The formula of having just enough scary to keep you on the edge of your seat makes the humor all that more enjoyable.
If I had some kind of a rating system this would easily be "You should already own it."