I have to go with WW, I was always sailing around the Sea to find islands, And every island had it's own secrets. I just loved that freedom

In SS they had the space to hide some big secrets, but there was really nothing to do up there. I spent some time on flying around but there weren't so many Islands where you could do anything. So I ended up just flying between skyloft and the next destination, like the sky was just unnecessary.
The original Legend of Zelda was surely great for exploration. So were TP, and OOT (yeah the field wasn't big but I loved the skeletons at night, and I also did a lot of running around in the Lost Woods)
But like I said WW wins for me =)