Ok, well I'll start out with what I think is easiest one:
Easiest: Twilight Princess would have to talk that prize on this one. Although I loved the new features, enemies, and especially graphics, I felt like I played through this :game too quickly, that may just be because it was too short for me though :S The second easiest one for me would be Minish Cap, it was just simply easy too beat to me. If you ask me I think Zelda should have difficulty modes, maybe unlock one after you've beat the one before it. But just like in The Legend of Zelda it would be a bit different compared to the first time you played it through.
Favourite: Majora's Mask I thought was the best Zelda game. I loved the different transformations, especially the Goron mask. I loved rolling around like that. And along with the unique transformations came the newer unique ways to defeat the bosses. Also, although it could be a big pain at sometimes I liked the new time factor in it, with all the side quests that required specific timing, side quests are one of my favourite things about Zelda.
Least Favourite: Adventure of Link....guess why? Yeah, maybe it's just because it is more different, like RPG, but I usually love RPG games. To me Zelda just couldn't pull it off. It isn't even because it was harder, it was collecting the experience, and the enemies were just boring to me. I did beat it, but I only beat it so that I could beat every Zelda game I own.
So yeah, these are just my opinions though....