Warcraft 1. I loved it even though I had already experienced a more advanced RTS in the form of Dune II.
I enjoyed Warcraft II as well including the expansion but I felt it was a bit too cartoonish and clichéd.
Warcraft 3 didn't interested me at all. Blizzard's games are too childish for my taste. I mean they got a type of childish that I don't like.