Ok, here's the hierarchy for me:
1. An open urinal, no "neighbours"
2. An open urinal, one "neighbour"
3. An open stall, door open
4. An open urinal, two "neighbours"
I dunno. I can deal with one dude because i can at least know if he's taking a peek by the way he holds his head up (always should be keeping eyes straight forward, if you ask me).
On a side note, this is quite interesting and related:
The Straight Dope: Aim for the back wall or the water?