This looked like a fun tag, so I thought I'd pass it along... it's hard, though. Some letters I have a ton of favorites for, but others I can't think of a single movie, so... here is my list of favorite movies from A-Z. Feel free to play along!
Arsenic and Old Lace
Catch Me If You Can
Dan in Real LIfe
Lion, Witch and the Wardrobe, Chronicles of Narnia
Muppets Take Manhattan
Robin Hood (Disney animated version)
Vita è bella, La (Life is Beautiful)
While You Were Sleeping
You've Got Mail
Seriously, I don't think I've seen a movie that started with a Q. If I think of one, I'll add it later.