Bonus points if it can be downloaded on GOG!
I’m super partial to the Age of Empires series. I’ve played it thousands of times both campaign and old school LAN party style, and even the oldest titles hold up extremely well. There’s just something about the way 90’s rts games were designed that never gets old.
I highly recommend Warhammer 40k: Dawn of War. In fact, the whole trilogy is worth playing, but the first game and the second are the best imho. If you like the lore and the overall look and feel of the 40k universe, you’ll love the games. As of now, I’m not sure if it’s available on GOG, however. the Master collection is said to be coming soon in GOG, but i’m not sure of a definite date. it’s available on steam though, and it’s quite cheap because of the sale. it ends on january 5th though.