I was going to pick this one. When me and my friends watch it we find something new in it every time that has us rolling.
The Man Who Knew Too Little. Probably the single funniest movie I've ever seen... and yet, there seem to be a lot of people who have never even heard of it.
White Chix, only because I thought it was going to be a stupid movie but ended up lmao..."Making my way downtown..walking fast...faces pass..and I'm home bound"
Super Troopers was pretty good. Which reminds me - Reno: 911 Miami was really funny, but I don't know if it was underrated. Patton Oswalt was so perfect in that movie.
I don't know if this counts: Best in Show. One of my favorites, it seems to be you either loved it or hated it for most people. I also agree Saving Silverman is underrated. They replayed that thing so many times on Comedy Central and I finally gave in after the 37th time. EDIT: One more...Don't Be a Menace to South Central While Drinking Your Juice in the Hood. I thought this was going to be typical crappy comedy, but it was full of LOL jokes.
I like Undercover Brother as well. There are stupid moments, but overall a funny movie. Another one in the same mold also with Eddie Griffin is Double Take
Norbit. I can't remember another movie that has gotten dissed more times than norbit in the past year. I went to see it and that movie was HILARIOUS.