Hoo boy, the Forest Temple was awesome, especially walking into it the first time. I really liked the Sacred Forest Meadow as Young Link, and it was awesome to revisit it as Adult Link. Then, to find the huge ghostly ruins of some castle or something that I never really looked at the first time there? Whoa! Awesome! "There's nothing so exciting as finding a secret in a well known place", and that's pretty much how I felt on finding the Forest Temple.
I really liked the whole atmosphere of the place, too.