Easiest for me: LoZ
Hardest: AoL
Personally, I don't care about completing games 100%. Collecting items gets really boring after a while. I don't know how many times I've started OoT with the intention of collecting all of the Golden Skulltulas, only to get caught up in the adventure and suddenly realise I've beaten a dungeon without getting them all. And then I can't be bothered going back. Maybe if I'm ever sick of playing the main adventure I'll aim to complete a game 100% and collect everything, but the awesome thing about Zelda games is their replayability. And when I'm not playing the main adventure, I like exploring on my own and stumbling upon things, and learning more about the land and the characters and things like that (i.e. Gossip Stones in OoT).
With that all said though, the main adventure of AoL is freaking hard. I am pretty sure I'd find it easier to complete another game, like TP or MM, 100% and collect everything in a shorter time than it would take me to beat AoL (and I'm saying that as someone who did beat AoL once!).