Do you prefer singleplayer or multiplayer.The command and conquer series is quite good for both if you like RTS .You can get the first decade for abot £18 very good value for money as it contains all the games over the last 10 years except CnC3 and its new expansion kanes wrath.Heres what you get for it.

Command & Conquer – August 1995
Command & Conquer – The Covert Operations – April 1996
Command & Conquer: Red Alert – October 1996
Command & Conquer: Red Alert – Counterstrike – March 1997
Command & Conquer: Red Alert – The Aftermath – September 1997
Command & Conquer: Tiberian Sun – August 1999
Command & Conquer: Tiberian Sun – Firestorm – February 2000
Command & Conquer: Red Alert 2 – October 2000
Command & Conquer: Red Alert 2 – Yuri's Revenge – October 2001
Command & Conquer: Renegade – February 2002
Command & Conquer: Generals – February 2003
Command & Conquer: Generals – Zero Hour – September 2003

I'd go Warhammer 40,000: Dawn of War. It's an awesome RTS, IMO. You can get the base game plus two of the expansions in one pack for like $20.00 now. It's the game that preceeded Company of Heroes and made by the same team. The Campaigns are engrossing enough to keep you interested but not keep you up till dawn. The Dark Crusade campaign has a Risk type map where each territory has its own bonuses and when you attack them you go to RTS mode. It also has diskless play. Its is simply a solid game.

I refuse to play Soul Storm. Is it true that they haven't patched the resource bug yet?

if you haven't played bioshock id get that, now

Warhammer is great!