Psi 5 Trading company... I loved that game... https://www.youtube.com/watch?v=Lp_uKS64LRE
Also from the same publisher: Card Sharks https://www.youtube.com/watch?v=4J7yE9ZNSwE
Unlike those guys, I was playing legit copies back in the day though :P
If it HAS to be a regular PC game... uh... pretty much all of the old ones were good enough to be "classic" today. Maybe Dark Reign since it had a bunch of weird/unusual things you could do for an RTS.