Low Spec
Warcraft 2
Low end, but has many unique features and TONS of custom units
Total Annihilation
Still has units being made for it and an active community, if you cant run SupCom then it is well worth a bash (and it is FREE)
Red Alert 2
One of the all time classics and is pretty much what all the later C&C games have been selling off of the back of (C&C at its finest here!)
Homeworld 2
A beautiful game that lacks many of the gameplay factors of other RTS games, but makes up for it in shear splendor of space battles and multiaxis attacks of space. Very atmospheric.
Medium Spec
Total War
Excellent use of strategy and unit formations, nothing does it quite the same. This is probably the ONLY RTS where you can actually plan an attack and follow it through to its completion.
Dawn of War + Expansions
For a beginner I would recommend this over all the others due to the variety of gameplay it provides. It is both humorous, fast and has the wow factor from some of its units. Although the game lacks the strategy of many others in favour of the 'RUSH' distinct with earlier RTS games, this will keep an RTS newbie entertained for hours.
High Spec
CoH
Company of Heroes has an excellent single player campaign and such a unique and intelligent design that you will be hard pushed to find anything superior
SupCom (FA)
SupCom because even though its single player campaign is short and pretty weak, it has a lot more variation in its AI and any half competent player will soon learn to exploit AI weaknesses. Make sure you get Forged Alliance (the expansion).
All of these games have skirmish mode for long lasting play, but some have an excellent single player campaign too.
Although I am a massive SupCom fan, if you can run it then get Company of Heroes because nothing offers more.
- Perfect Singleplayer campaign
- You can employ some amazing tactics and strategy
- Excellent units with a large amount of variety of customisation
- Cover system (Yes an RTS too!!!)
- Great looking and support for DX10 effects
- A free expansion
The only downside is the HUGE downloads required to patch it from scratch!