Jane, I agree about Groundhog Day. I've probably seen it at least 20 times....not always in its entirety but bits and pieces. I just can't resist watching it whenever I come across it on TV.
Back when I first saw It's A mad, Mad, Mad, Mad World I thought it was the funniest thing ever made. I find it less funny now but at the time it was number 1.
Other faves:
Blazing Saddles
Animal House
Fish Called Wanda
Throw Momma From The Train
High Anxiety
Any Monty Python flick
Christmas Vacation
Ghostbusters
What About Bob
The Hope/Crosby Road flicks
The Nutty Professor (Jerry Lewis)
Airplane...just to name a few