Wow this is tough for me... I absolutely LOVE John Hughes' movies!!
I would probably have to say that my favorite is The Breakfast Club, because it always seemed so real to me. There's this group of kids that are so different on the outside, but when they're forced to spend an entire day together, they come to realize that they are all the same on the inside. They share the same experiences and the same feelings as almost every teenager across the country.
And even though Hughes wrote Pretty in Pink, but didn't direct it, it's probably my #2 selection. And I always get great laughs from Ferris Bueller's Day Off and Sixteen Candles.
I could go on and on, but I won't! ;-)