I've always liked the forests of the Zelda series, especially the lost woods. In TP the lost woods were just amazing, but my favorite forest has to be the OoT lost woods.
Either the Field (Mainly TP0 or the Swamp in AoL. I love those ares. Now, I used to hate the swamp, but when I realised I could get (slow, mind you) experience gains, I ended up going there a lot.
The field is the best battleground, next to being inside. I'd say for that reason Hyrule field is the best place to run around, and it was justifiably put in there as the central hub for the Zelda games(excluding Wind Waker and PH)
Field for me. Just kind of the memories of riding around on Epona for the damn great music that is played out there. It's nice and open too, I like big.
I've always loved the fields in the Zelda games. That's where I spend most of my time. Especially after I beat a game. Second would be the water areas.