Unreal Tournament 2004 (or the original UT I suppose)
Comes with Unreal Editor
Has a number of default maps geared towards various player counts (plus a ton of downloadable custom maps)
Supports LAN
Quite fun
Only caveat I'd mention is that the online multiplayer doesn't have nearly as many players playing as it did at its height, just a couple deathmatch and onslaught servers left. But for LAN I'm sure you and your buddies would have a good time. There's a demo available for UT2004 (and the original) if you want to try it first.
Looking for an FPS that meets these standards