The easiest for me would have to be the Spirit Temple. (Out of the five dungeons you go to as Adult Link)
While many say that the hardest one is the Water Temple, I have to disagree. While it is kind of tedious and challenging at points, I don't think it's the hardest one. I have to say the hardest temple for me was the Shadow Temple. In fact, it really isn't so much hard as it is annoying. Mostly because it's very easy to get knocked off the platforms and into pits. Not to mention there's one room in which there's a fireball that flings towards you, and you only have a split second to block it before you get hit. Which I find really annoying. And if you die at the boss, you have to make your way through the whole temple again. Including the damned boat ride.
Seriously, I almost gave up on the game due to how annoying that temple was. Thankfully, Gerudo Valley and the Spirit Temple weren't nearly as annoying and were in fact, pretty fun.