Actually, for gameplay, I like WC3 and WC4 the best ... WCP didn't have enough variation or surprises IMO, just lots of "Defend X" and "Destroy Y." No nebulae, no stealth, no planetary missions, very few "choose your destiny" situations, etc. (The only ones I can remember are choosing between the bomber or the interceptor at one point, and choosing to either kill cats and watch Hawk die, or not kill cats and hear about Hawk's death later.)
WC1, 2, and Priv were fun to play, but the asteroids sucked and were everywhere.
However, for storyline and general coolness of "look and feel," WC1 and the Secret Missions blow the rest of the series away. Those moody oranges, that stark red, the Captain looking out of the viewports thoughtfully before speaking to you, the subtle pathos of a leaky pipe dripping in the crew quarters ... just awesome. I still pull out "Claw Marks" and read it for pleasure, which I don't generally do with "Victory Streak."
HOWEVER,: For overall coolness of concept, I'd like to give Privateer an honorable mention. "Traveller" meets Han Solo, set in a universe cooler than both of those. What's not to like? The only problem with Privateer was that there wasn't anything near _enough_ of it. Just when you feel like you're starting to really get going, the story's over ... and Righteous Fire, while a pleasant add on, is a bit weak storywise.
OTOH, I've never played Privateer 2, because I was under the impression that it wasn't set in the Wing Commander universe. Am I misinformed? If so, it's time to hit eBay...
-The Gneech