A lot of people are probably going to disagree with me on this since it's a pretty close call between quite a few epic movies... and then this.
Anyway, I'm going to nominate
Bob Whoops' long run of Umihara Kawase. I'm not sure if having never played the game beforehand helped or hurt my enjoyment of it, but I definitely got a kick out of watching him catapult a little girl all over the place, and provide entertainment during the otherwise long boring boss "battles." I loved the frog and crab wrangling he did, along with bringing himself to the brink of death numerous times only to slingshot back.
In summary: not too long, not too short, varied action with very entertaining waiting periods, and unfamiliar subject material (on a side note, a friend and I each ended up losing about a week of our lives trying to play through the game after watching the movies in the speedrun competition thread) add up to equal my (most likely meaningless) vote in a very heated field.