Well, first off. SFC2 vs. OP. OP is the "stand-alone" expansion. Now many didn't move on because they had their undies in a bunch over the "stand-alone" thing. But in the end OP is everything SFC2 is and more. More ships, weapons, I think arcs, etc. The next patch looks to finally vanquish the vast majority of bugs that was holding OP back from being the stand out, so there is really no reason to go for SFC2 over OP at this point, especially since you can download the single player campaigns from SFC2 and plug them into OP. It's really a cake and eat it too thing.
As for why OP is best, the gameplay for me is just vastly superior. It's got a nice pace to it, nice tension buildup before the first strike comes in, and I like the stalking eachother type of battle. Very cool. Not to mention the game has superior diversity over other versions. It's got lot's of race specific arcs and weapons that REALLY make playing different races different. You cannot fight race X the same way you fight race Y, and you certainly cannot fly race X the same way you fly Race Y. The difference between say the Hydrans and the Romulans is amazing. Racial diversity is at it's peak in OP.
Beyond just the superior tactical play, the game has visual diversity. Something that was lacking in SFC3, imo. Every race has their own screens in tactical, as well as sounds, so that you really get into flying a certain race. You feel like you are really part of the game, commanding a ship. Now, SFC1 took the visual diversity further in having different campaign interfaces for each race, which was too cool, but at least you have different racial controls when you are actually fighting in SFC2 and OP.
Now, there are some tweaks that are nice in SFC3, especially a few options in the D3 vs. D2, but for my buck it's OP all the way. I can live without those couple of features in exchange for the superior gameplay and atmosphere. I've started playing OP more and more recently (again) and every time I boot it up I just get euphoric. It's too cool (but that said anything can always get better....
) OP is not only worth the 9 bucks from the bargain bin, it's a steal. It's highly unlikely, imo, that you will ever find another game of that quality and depth of play (post patch) at that price or any other.