Hard to say - both Starflight games are excellent story/exploration games, but very, very dated in appearance and engine (i.e. clunky ui, very basic combat system). If you can get past that, play Starflight 1&2 in order (similar design, a continuing story, definitely worth playing both). Then play Star Control 2, which has almost identical game play to Starflight, but with much better graphics, interface and engine.
Star Control 1 is pretty much unrelated to 2 and can be skipped entirely - or just played as a separate strategy game with no real links to the sequel outside of the combat engine.
You didn't mention Star Control 3, but just in case - it feels like an attempt to combine SC1 strategy with SC2 RPG elements. Unfortunately it doesn't really compare with SC2 - not a bad game as such, just unlucky enough to be a sequel to a very good game.