The old railroad. That is pure nostalgia for me because of how much it sprawled across the map. Having a friend GameCenter summon(remember those?) me to the pet shop on the cliff below and to the left of Sardon since I didn't have a drasine then.
That area to the left of burger refuge also had some trail track I believe, I remember tons of people using the rainbow morphs in there. The old cave in deadwood where you could hop on top of the walls and walk around(that was funny. Only a single area on the map that that's possible now I believe).
The old password treehouses were a blast, I really miss those. One was to far right of brothers, I forget where the other one was. Oh yeah, near Jesus Freaks I think. I also remember seeing Azrael's club for the first time. That was a blast.