yup deffy european wars: cossacks - art of war or later expansion/version. 12k max units (!) huge maps, far bigger than you have ever seen in any other RTS. its RTS as it should be but with inferiour gfx (its old)
UFO: enemy unknwon (also known as x-com) is a winner as well. 1, 2, 3 and the newest. other releases = shit.
another game with same type of gameplay is jagged alliance. the first one was for DOS. they recently released a jagged alliance online. im playing it atm (steam version). seems good. its F2P but if you like the game, id say you should spend the money (about €13 AFAIR) and pay for the extra campains and get access to more maps and gear.
naturel selection (HL mod) if you have internet on that old pc
and last but not least: whats wrong with counterstrike 1.6?
heroes of might and magic whatever version, i like them all.
settlers
civilization
damn i cba writing them all haha