I couldn't possibly come up with a single favorite PC game, but I love sims and RTS and both genres are best enjoyed on (and mostly developed for) PC... Games like Company of Heroes, Total War series, Age of Empires, Empire Earth on the RTS side and Football Manager, SimTower (old school!), RollerCoaster Tycoon, Prison Architect, Democracy, etc... on the simulation side.
Other than that, two of my all-time favorite games are also PC games: Killing Floor and Operation Flashpoint: Cold War Crisis (and expansions).
The former is a co-op wave-based FPS where you pick a class and kill zombie-like monsters with a team of up to 6 players and earn money to buy weapons and gear. It's a very gratifying FPS with that progression a la RPG with such a unique take on the concept and very addictive and fun gameplay and mechanics. The sequel is currently on Early Access and it's everything any long-time fan wanted the sequel to be.
The latter is the prequel to the now-notorious ARMA series, and in my opinion, much more fun and engaging than its successors (not that they're not amazing games, mind you). Despite being quite dated by now (it was released on the year 2000 and the expansions on 2001, if I'm not mistaken) it still holds up quite well and it's still unique enough that playing it is an absolute blast. It's also very remarkable how much the modding community is alive and well even today. I made scripts and worked on a mod team in 2004 or so and it's amazing seeing the same people still releasing stuff 11 years later. The campaign for the Resistance expansions is still one of my fondest gaming memories and was way ahead of its time; there's games still trying to do the same thing poorly compared to this low-budget game from over a decade ago. I would most definitely recommend this to anyone with a PC and even mild interest in video-games, but especially to the types who enjoy building, like Minecraft/Terraria players. The map editor on this game is also to this day a concept unheard of in FPS/tactical shooter games.