The "best" is kind of hard to quantify. In terms of playtime, I got the most mileage out of the Dawn of War games (still need to get Soulstorm at some point). Lots of different factions to master. I didn't really care for the campaigns in stock DoW or Winter Assault, but I had a lot more fun with the Dark Crusade style campaign, which was more open-ended.
Coming in second would be Civ 2, which I put more time into than all of the rest of the Civ games combined. And third would be Company of Heroes / CoH: Opposing Fronts -- same engine as Dawn of War, but some different gameplay mechanisms and different feel (not to mention a WWII theme instead of sci-fi).
Coming in second would be Civ 2, which I put more time into than all of the rest of the Civ games combined. And third would be Company of Heroes / CoH: Opposing Fronts -- same engine as Dawn of War, but some different gameplay mechanisms and different feel (not to mention a WWII theme instead of sci-fi).