Toriver said:
Hardest: Water Temple. As was mentioned earlier, more tedious than actually difficult, but damn, that last small key...
Easiest: Spirit Temple. Or at least it always seemed like a cake walk to me because I would always do it right after the Water Temple. Getting through Gerudo Valley to get there is harder than the temple itself. Though I have an interesting question. After the Water Temple, you're basically left to figure out for yourself where to go next. Are you officially "supposed" to do one of the Spirit and Shadow temples before the other, or is the order meant to be flexible?
I'm pretty sure you need to do the Shadow temple first. As the Lens of Truth is required to get through the Desert leading to the Spirit Temple. Unless you somehow memorize the path you need to go to get there.
Also, doesn't Navi tell you where to go next after each temple?
EDIT: Actually wait, never mind. You get the Lens of Truth in the well. So you can technically do the Spirit Temple BEFORE the Shadow Temple.