Almost all of my favorite films are comedies, so I can't narrow it down. Ghostbusters is my all-time favorite film, so that takes the top spot, but the rest in no order...
Back to the Future
Back to School
Caddyshack
National Lampoon's Vacation
Beetlejuice
Ladybugs
South Park: Bigger Longer and Uncut
Army of Darkness
The Naked Gun Trilogy
Brain Donors
Dumb & Dumber
Ace Ventura: Pet Detective
There's Something About Mary
Clerks
Mallrats
Dogma
Chasing Amy
Jay & Silent Bob Strike Back
Jesus Christ seriously way too many. I love toilet humor, so that's where most of my comedies lie.
As for classic cinema, anything from Laurel & Hardy and the Marx Bros. Paramount films for sure..
Speaking of classic comedy, off-topic, but anyone else like Joe McDoakes?