I guess I'm in the minority of people that never played SC2 but really enjoyed SC3. Something about SC3 really clicked with me. Maybe it was the only-half-serious politics storylines between the races, or the planet/fleet development, or the mystery solving, but I really got into it and have played through it several times.
SC1 was one of my all time favorite early DOS games, but I did not have the hardware to play SC2 when it came out, so I never got around to playing it.