I'm gonna dare to be different here.
Space Combat flight simms are my very favourite, followed by really good RTS, then FPS (FPS is fun but I'm really not that good at it).
please note though, the RTS's have to be REALLY good. A really good RTS is excelent, but a bad one is just Dune II over again and I played that out back in '95. I can't stand Dune II clones! (inc C&C)
My train of thought is a runaway!!