Starfleet command is the only one of those games I have...
SFCI
If your talking about SFC the original, its very good. If you ever played the pen and hex sheet game its based on, I'd call it a must buy. Otherwise, I'd just skip to SFCII for the Online dynaverse:
SFCII
Added a few more races and something much, much better: a persistant, Online universe where you all fight for control of quadrant. There are a couple servers with as many as 40 players on at a time. You pick a race to play on that server and take over the hexes by doing successful missions against it, some against other players. This alone makes it worth it, imho. Plus most races have many missions to go through, with race specific missions that have multiple storylines.
SFCII: Orion Pirates.
Same as SFCII, but with some new races (all pirates) and xships (powerful ships). If you buy OP, you can't play on the persistant SFC2 servers and vise versa, but there are around the same number of OP servers. SFC2 has a better single player experience.
SFCIII: The Next Generation.
They just announced that SFC will no longer be based on the SFC series of board games. They have a license to use the Next Generation storyline and ships and they are "streamlining" the play (4 shield facings instead of 6 for example). To be released this fall/winter.
In short
, I love Star Fleet Command. I still play SFC2 and OP even years later.