It's a close one between the Cucco Ranch and the Octoball Derby for me. I love the Cucco Ranch, I just started laughing the first time I played it, and now I am playing it constantly in an attempt to get to 999 seconds (that coveted Large Cucco >_>). I love baseball, so naturally the Octoball Derby was immediately a favourite of mine.
I ended up voting for the Cucco Ranch. I did because I feel it has more depth to it, with four different stages to try and one that goes on for almost 16 minutes >_>. The Octoball Derby, once you get 100 Rupees and the Piece of Heart, there's not much to do. Sure, it's an easy Rupee grinder, but so is everything else in the game, so I never found myself needing to play it in order to get more Rupees.
My current record for the Cucco Ranch is 343.89 seconds. Over a third of the way to 999...............