Lobby balcony is the best spot. When its packed, you are gonna have to watch the stairs every 2 seconds on the sniper houses. The only problem with the lobby balcony is there are like 3 spawns you are potentially raping the whole time. I agree with wally that, for the actual purposes of a sniper, S1 sucks. You can't actually play the role of sniper on s1, people consistently spawn behind you and you have to do close quarters combat almost the whole time. Also, for the most part, if you sniper someone from a sniper spot, the person you killed is gonna come looking for revenge. If you rack up 3 or 4 kills in the same spot, you will be anti-camped by someone looking for revenge, just like any other camper would.
For me S1 is an overadjustment for my sniping. You shouldn't actually be playing like that in a real game. I mean running around, scoping people up close is kinda ridiculous, and will most likely get you killed. If you want to win on server 1, don't snipe. If you've decided that you need to prove to everyone else that you can win with a sniper on server 1, then that's fine, too. If you manage to do it, that means the rest of the people playing are either not very good, or got very unlucky, assuming you played legitimately. Wally is right, playing with sniper on s1 is not ftw.