Later iterations, like Scorch and Worms, added a variety of upgraded weapons and terrain that could be destroyed, but a simpler, space-based version has been my favourite for years.
In Gravity Wars, you view the action from a top-down perspective, as two spaceships alternate firing missiles at each other. As you can surmise from the name, the gravitational pull of nearby planets can have a pronounced effect on the path of your shots, which varies depending on their density. It all sounds very intense and slide rule-ish, but in reality, the game is surprisingly intuitive, and both of my girls enjoy playing it quite a bit.
I've played it on my Amiga and a variety of PCs, but I'm currently enjoying it as an app on the iPad (although it's made for the iPhone). They've dispensed with the numerical entry in favour of a simple but effective graphical interface that makes the gameplay quick and easy. I love playing it with Fenya and Glory as we can pick it up and put it down easily, and playing multiple rounds takes a long time to become tiresome, as every new engagement is like a puzzle to be solved as you work out your firing solution while the enemy shots draw closer and closer. When they start studying parabolas and the like in math, I hope they recognize them from this game.
Maybe there is a lesson to be learned as well about the fickle nature of ballistic projectiles, as illustrated by the near miss I did to myself in the picture above. Or even more spectacularly in the picture below; my second shot of this particular game, and the shot took 3-4 minutes to resolve.
I haven't yet witnessed a non-terminating shot, but I have to think it is possible for a shot to enter into a stable pattern and orbit the area in perpetuity, which I think you would have to treat as a complete do-over.