One of those hard questions for me. I'm not very user-friendly with most comedy. The movies I find funny aren't actual comedies. Hilarious implies one I'm laughing at constantly and I'm not sure I can name one.
Er, this is probably as close as I'm going to get.
Before 1970: I do love It's a Mad, Mad, Mad, Mad World. And I love What Did You Do in the War, Daddy? I love the Road movies, particularly The Road to Bali and The Road to Rio. The Princess and the Pirate is also highly amusing.
Post 1970: Well, A Life Less Ordinary is right up there at the top of the list, along with Free Enterprise and, I have to admit, Madagascar 2: Escape to Africa. I thought I would rather hate the latter, but after watching it a few times with my little nephew, the sheer absurdity of it won me over and now I love it. The awesome crazy penguins and Alec Baldwin's presence help.