Well, Adventure of Link (Zelda II) was probably the worst but that's because it was so much different then the other titles (in my opinion in a bad way). It was filled with strange comments that didn't make much sense; the walking around collection stuff got tiring, and the enemies were really hard (esp. after hours of playing since there was no save function back then). But after that I need to say Windwaker and Sprint Tracks might be the worst - I still beat them and enjoyed them but in comparison with the rest these games were less fun, mainly again because of the repetitive traveling (sailing and train ridding are not my cup of tea apparently).