Eldin was the toughest only because I didn't have a set plan when I first attempted it. I purposely failed faron to collect dusk relics. Skyloft was easy if you are patient and use the terrain as well as you can.
In Eldin the tears were kind of hidden apart, but I died a few times in skyloft right before the end, and that pissed me off. So, I'm going with skyloft.
I don't really know which had more "death" counts--Nayru's trial (Lanayru Desert) or Din's trial (Eldin Volcano), I had difficulties with both of them.
Skyloft was...difficult due to the oh so many Watchers guarding a single Tear, hard, but it took me only 3 tries before I figured something suitable for me. I voted Lanayru Desert trial since it seemed to take more time than Eldin Volcano. I think.
The hardest for me was definitely Skyloft. There was that one piece that was on top of that box which was on top of the Knight Academy and it was surroned was the lantern ghosts. It always took me a long time to get, enough time for the seconds to expire from the last Tear of Light and then enough time for the Guardians to come kill me. The easiest fo me was Foran Woods. The first time i did it i beat it really quickly and easily.
Skyloft hands down. The space was very tight and the lanturn/bell guys were placed in very tight spots and had vision out of their lamp light this time (that one in the graveyard caught me everytime!). This was the first one that I actually failed, it took me 3 tries to beat it and the third time was a lucky grab with a guardian right about to slash me as I grabbed the tear.