Released (I think) in 1999 originally, I highly recommend trying out Warzone 2100: https://wz2100.net/

The code has since been released as open source and some great work has been done improving the game, in terms of graphics, gameplay and add ons (of which there are a great deal). It features an offline skirmish mode of course.

I was always a big fan of Age of Mythology. An updated Extended Edition is available on Steam.

For single player, Warzone 2100 (it's free and has been greatly enhanced by a devout fanbase) and the Earth series. While I never really got into the Earth games, WZ2100 is one of my favorite games of all time - unit customization, several hundred research projects, a great atmosphere and really challenging gameplay, even if that is often caused by dated AI and pathing (they may have fixed some of these issues in the fan version, though - haven't played it in a while).

Well, I got Tiberian Sun running but it felt really dated to me. Actually the graphics were fine (although the menus look like something I'd do as a place-holder) but the UI of having a list of buildings and a list of units seemed so ancient :) I just jumped right into a skirmish so may have missed some nuances but it reminded me a lot of the reason I started making my own RTS, frustrated by how limiting the UI was in the early RTS games.

I think I'll check out WZ2100, sounds interesting. Or maybe C&C3 although RA3 was a bit "too 3D" for my taste, if that makes sense?

So far, WZ is looking pretty neat. Nicely retro. Although my "try playing without reading the manual" didn't go very well, as I was stuck how to create any units except trucks ;)

I'm also a big C&C fan (and dune 2), I can remember going to a shop daily after school when Red Alert was about to be released and asked whether the shop finally has it (1996 *yay*). I really wish the genre had evolved in more military way.

Beside that I've also enjoyed "Stronghold" 1&2 (not the newer 3d versions), not really the setting I like, but I got it as a gift and it turned out really fun. It's not just war, but also a lot of base building.

homeworld was really cool, but mainly for the 'epicness' of the story, not for the gameplay, the actual gameplay

I've also enjoyed start trek armada.

(I've also tried turn based games, Advanced Wars was sometimes really nice, I remember one mission was so damn challenging, I've started to play it with full batteries and by the end of the mission the battery light indicated it was low.)

I never really tried turn-based games. Are there many games which merge RTS and turn-based elements well?

games which merge RTS and turn-based elements well?

Total war games are probably the most known for combining RTS with TBS