Plants vs Zombies on PC.

Great, unique, iconic, still fun to play. Its biggest achievement: I have brought a lot of people into the hobby by making them play this as their first video game and there wasn't a single one not having fun. Tower defense is as a whole an underrated genre if we talk about the best games of all time. It also is a game that offers achievements that add a lot to the gameplay by challenging you to change your tactics.

They of course had to make the second one mobile only and on top ruin it with microtransactions. :( Greed is why we can't have nice things.