It would either be Eldin or Lanayru, but I haven't done the Skyloft one yet.
In Lanayru, the patrolls kept catching me and I died, and of course the quick sand made it pretty hard. Also, I had a "blonde moment" because I had one more tear left and I had to do it a couple of tries to find that it was actually ABOVE the Lanayru Mining Facility.
The Eldin volcano was very difficult at the part where you had to run up the slopes, avoiding the patrolling ghosts. They caught me a lot of times, and I had to start over. Again, after a while, I only had one tear left, and with only 45 seconds left, I discovered it was on the sliding slope part on the other side of the area where I was. I eventually reached it, but I only had a couple of seconds left. I knew I only had one shot, but my luck kicked in and I missed it and ran out of time. I was still determined though, so I was dashing up the mountain again to the slope with the guardians close behind my back. When I slid down, I missed the platform again. So I went back up again, and I finally got the platform. It was a lucky moment because the guardians never got me. It was so frustrating and brain wrecking though, so I am pretty scared to see what Skyloft is like.