If it's a question of world-building I cannot go into much detail as I never really played many of these besides Roller Coaster Tycoon 2 (which I must say was a pretty solid game) If you are more in the battling side of RTS; I think part of the question is what genre you are into. As far as modern warfare goes I would say, hands down, the C&C Generals series takes the cake. If you are more into future warfare; Starecraft is easily the best in that genre . It's also the most advanced when it comes to in-depth game-play. In you are all about the fantasy world; then Warcraft is the RTS game for you. However the actual game-play is kind of simplistic.