William and Sly Top Notch
It's extremely rare to find a flash game like this. Smooth controls, great music, wonderful graphics (the parallax scrolling was done very well), and an all-around fun experience. The concept and story are simple, yet it's refreshing to see something that wasn't bogged down by details. An all too common mistake of many flash game makers is to create a storyline more complicated than a big-budget RPG for a game that takes less than an hour to complete. Another notable feature is the lack of two common elements of flash games: annoying glitches and awful physics. This game sets itself apart by actually having good physics and no noticeable glitches.
William and Sly is everything a flash game should be, and it sets a new quality standard for such games.
As a side note, I want to add that I instantly fell in love with Sly despite the lack of character development. How can anyone not love a cute little red fox?