You seem pretty confused, aren't you? OK, here are my suggestions:

- Civ3. Great gameplay, abstract battles, loads of mods. If you get it, go for the full package (with Conquests Expansion) and visit some Civ3 site (like Apolyton or Civfanatics) to find a whole world of mods, units, scenarios etc. etc. Addictive. SP is the best experience of it, MP is not really that great. Alpha Centauri (Civ in space, sorts of) never did it for me. Many oldtimers prefer it over Civ3, but not me.

- MTW VI. Go get it, a great game. I consider RTW much, much better (in the strategical aspect, at least, in the tactical it has serious problems) but MTW VI is the absolute for MP. And if you want a really extraordinary SP experience, install MTW VI with Wes' Medmod, the greatest modification ever for a TW game.

- Anything by Paradox. If you want a solid SP experience and don't care much about battles, you can choose between three gems: EU2, Vicky and Crusader Kings. Have played all three of them (in the last 14 months I only played those three and RTW, and recently Mount&Blade) and they are incredible games. Vicky has the steepest learning courve of any game I've seen, you'll have to read some stuff about it before you start playing. EU2 is much easier and you can figure out how to play in an hour or two - for "how to play properly" it'll take you much, much longer though. CK is more simple than those two and is recommended only if you are into the Bene Geserit trip.