Fly is still my favorite episode. I've watched it so many times and I never get tired of it. Walt failing at killing the fly and then when he sneaks up on Jesse like a horror movie gets me every single time
Plus everything that episode stood for - the fly representing Gus and how Walt couldn't kill it without Jesse's help, just like in S4. Also I think it was the crossroads in the series for the Walt/Jesse relationship and Jesse's character. 62 episodes in this series and Fly was the 30th episode, so almost exactly the halfway point.