I think out of every series I've watched, the one that stands out more than any other in my mind for sheer enjoyment of watching was Azumanga Daioh. I don't really understand why, myself. It's just .... really good!

Second place would be the Read or Die OVA and third would be the Cowboy Bebop Movie. Akira and GITS (movie) tie for fourth on my list -- because akira was one of my first (and how can you compete with a solar orbital laser?) and gits just had such an interesting plotline and nice shades of depth behind the technobabble.

But really it just depends on the mood I'm in.