The Best 1 player, old game right now for myself, is Evil Genius.

hmm. I've never played Dune 2 or Emperor. I'll have to try and get a copy of Emperor then.

Though, have you read any of the Dune series, and does Emperor match up with the books? Note that Dune 2000 (and likely Dune 2) do not match up. Like, barely, if at all.

Emperor is similar to Dune 2000 and Dune 2, it's set in the Dune Universe, but doesn't follow the events of the book. The story is kinda camp, like the Red Alert series, however it's still fairly good. Worth a play through if it's not too expensive.

Allegiance.  Space-shooter, all multiplayer.  Basically each player is a unit in an RTS, with one player the commander telling the miners where to go and where to build bases.  Meanwhile, he tells his players to escort this, scout that, or bomb here, but players don't have to listen and an do most of that independently (you can't build a bomber yourself because it costs money).

www.freeallegiance.com 

Master of Orion 2

Gradius 3

R-Type

Wing Commander: Prophecy

Tropico

any game in the Warlords series

Fallout and MDK Series.

Call of Cthulhu: Dark Corners of Earth - action FPS adventure with few RPG aspects and fantastic atmosphere, normally i do not play this kind of games, so when i recommend it, it is really worth to try

Star Command Revolution - great space RTS from 1997, i never needed DOSBOX or anything to run it under XP, but my friend werent so succesful, so maybe you might need it too. Again, the game is absolute gem with IMHO the best SP campaign alongside Homeworld. It is pretty tough game as well, where in consequence of spending too much resources during campaign you can few "missions" later find out, that you lack these resources to win the mission and therefore finish the campaign.

Medal of Honor Allied Assault - IMHO best multiplayer FPS ever