My favorite add-on for this would have to be DayZ.
Just kidding.
But really, it would have to be
Advancing Power. It's a pretty much hold-the-line-while-pushing-it-back kind of thing. You define two bases on any map you want with an initial "fire zone" which is where both sides will run to at the start of the battle. You can have up to 12 different "main" groups of different factions with a bunch of "background" groups of either infantry, light vehicles or armor. You can call in all kinds of support like artillery strikes and radar, and when you kill something you get money + points that can be used to buy new soldiers, vehicles or weapons. It's plenty of fun. The AI likes to be a richard, though. I set the battlezone to Chernogorsk with the bases outside of it and the main fire zone inside, when OPFOR decides to call in a Spetsnaz paradrop right outside the city in the field where BLUFOR spawns. 6 personal deaths, several BLUFOR artillery strikes and a lot of dead Spetsnaz later we finally cleaned up the paradrop and went along with the battle. I love that add-on.