I voted for the Oracle of Ages, since I think that game is the hardest Zelda game (other than Zelda II). I always get lost getting to the next dungeon, and the insides of the dungeons have tricky puzzles also.
I've voted for Spirit Tracks. Because I'm currently playing it. But any new Video Walkthrough would be really nice. I love them all. But I have to admit, I'm a little addicted to these Walkthroughs. I have to change this attitude. I have to play more by myself in the future, just like in the old days ;D