Best Funny Movies

OK -- I'm in need of a funny movie -- did some 'Googling' of 'Best Funny Movies' lists, looking for something I haven't yet watched.  One list had This is Spinal Tap' as their number one choice, another list picked Monty Python and the Holy Grail and another one deemed Ghostbusters as the funniest movie of all time.

Right now, I'm leaning towards Ghostbusters since I've never seen it all the way through.

What are your suggestions?