To be honest, I thought the Great Bay temple was easy. I voted for the TP temple, because it is the only one I had to use a walk through for. The water temple in OoT, was hard, but I've beaten the game 7 times, so I kinda got used to it.
I walked into the Great Bay temple scared, because I figured it would be extremely hard, but it didn't take me that long. I actually had a lot of fun completing it.
The water temple in Oot was very hard the first time, but It just kept getting easier for me.
Finally, the Lakebed temple in TP was hardest for me, because I had to ask a lot of people for help, even though I was only 10 at the time, I still think I would have a hard time with it now, 4 years later.