The only one on the listed series I haven't played is AoE III. RTS is probably my favorite genre tied with RPG, and I definately have to go with Command & Conquer. I think Generals might be my favorite RTS so far, I honestly have to say the way that each unit can be countered is done on a level I haven't really seen, at least not in the others on the list. On the others you can really spam one good unit and murder anyone, but in Generals any spam can be demolished rather inexpensively. I also think it really shifted the basic focus of RTS from economy to pure strategy. In the earlier days I definately preferred C&C to WC and RA2 to SC. So yeah, C&C.