I'd guess Chamber 15. The double-fling is a PAIN to get.
Speaking of Portal, I really need to finish my papers so I can go play the sequel.
EDIT: Hey, Mak, here's a tip for using the fling technique in Chamber 15: Shoot the blue portal up at the top of the room, then shoot the orange portal on the wall. Run through the portal, and shoot the orange portal on the floor right when you land. The segment where you have to place your portal and run into a side chamber to do your initial jump actually requires this.
To sum up: whenever you have to shoot a portal on the floor, try to do it in mid-air, right before you land.
