My vote probably goes to one of Studs' battles. Unfortunately the links on our Archive page no longer work, but while they were up the Battle of Tank Stream was one of my favorites. To put it in perspective, at one point an armored vehicle dropped a squad of about ten heavily-armed Spacemen into the fray, without warning, and the players on both sides basically ignored it. The text implies that by the time the defenders had wiped out the attacking force, about half of their army was still just sitting there behind their fortress wall. And that's one of Studs' smaller battles.
If you want some more accessible battles, though, BLOC has a couple of gigantic ones. Check out Death to the Pepsi Brotherhood and the one immediately after it on the links page.