A Passport To Pimlico, as I love the Ealing comedies. I also love Margaret Rutherford films (such a dotty lady, who always makes me smile) and this film covers both of my likes.
My all time favourite film is Fight Club, but I guess the one film that can alter mooRAB (for the better) for me would be High Fidelity starring John Cusack. That film leaves a smile on my face everytime I watch it.
I saw 'Carry On... Nurse' the other day and it's just the perfect kind of film for a weekend matinee. It also dawned on me how good the early 'Carry Ons...' were; some of the later films got a bit tiresome after a while...