ANY C&C GAME BEFORE C&C 3 (C&C Red Alert 2, C&C Generals)
C&C 3 is a fairly nice game. Everything beyond that is terrible / meh.
I recommend getting Wargame: European Escalation if you want some more slow paced strategy game. This game is however mostly multiplayer based.
Wargame is a really unique : You get to command a stuff ton of units resembling real life units. The game takes place during the Cold war and simulates ground combat between the NATO and the Warsaw pact. The game also features no base building of some sorts.
It's more realistic than other games for example your units run out of ammo and fuel so you have to send in supply trucks for rearm and refuel. You also have to send in recon units as the maps are very large too ( up to a few houndred square kilometres depending on map ). The graphics are very amazing for a strategy game. As it uses an improved version of the engine used in R.U.S.E. you can zoom in very close or very very high up.
With "a stuff ton of units" I really mean there are very many units - about 300 different kinds of tanks , helicopters , infantry squads , APCs , recon units and more.
The game isn't cheap but from time to time it gets heavily reduced. Just a few weeks ago it was reduced to $12.
Check it out on steam