Well I don't think Starcraft can be considered a "series"... yet.
I loved Dune 2, it was my first RTS (Dune 1 was cool too, although it wasn't "classic" RTS).
I played Warcraft 2 and it rocked.
Command & Conquer was good, but it never really "conquered" my taste.
I liked a lot, a very large lot, The Settlers series, and I think it should be in the list.
But I still think Starcraft was the best. It simply fascinated me. The story, the ambient, the character... everything.
1. Starcraft
2. The Settlers
3. Dune
4. Warcraft
5. Command & Conquer
6. Age of Empires
(Last place doesn't mean it's not good - I love RTS'.)