There is no "first ODST level", but if you want what they intended as the first, it's the Plaza (playing as Buck).

As for Sierra 117, I get a kick when I have a Needle Rifle and Needler and can efficiently clear the entire canyon in a couple minutes WITH some assassinations mixed in. Winter Contingency was awesome because the layout was perfect for setting up assassinations and you can precisely dispatch Grunts with that over-abundance of assault rifle ammo.