A mix of tower defense and RTS, Creeper World 3 is radically different than the rest of the games on our list, but still well worth playing if you want to try something new.
Instead of hordes of orcs or stormtroopers, here, you are basically fighting a spilled liquid that terraforms the terrain. Your goal is to stop its relentless advance and eventually fight back to take out the source. It's somewhat like Plague Inc., with a nefarious virus-like substance polluting the land.
There's a lot more strategy to it than you might think, though, and the number of levels and playthrough styles on offer are staggering. With a level editor included, you could easily sink a very silly number of hours into mastering every aspect of Creeper World 3.