So, I am extremely hyped for Star Citizen, so I've been wanting to try some of Roberts' earlier works. My question is, where, in your guys' opinion, is the best place to start? Which Wing Commander should I buy?

Honestly the entire series is great, starting with the first game. The mission design in the first game is top-notch, and there's some really great emergent cinematic gameplay in the sequel. The 2D graphics in the first two games haven't aged terribly well, however. If playing games with older sprite-based graphics is hard for you, then Wing Commander 3 is a good starting point. If you can handle them, then go ahead and start at the beginning!

For me, the second game, but I like old school games and am yet to play the third so I am not super qualified

I'd say start with any of them except IV. WC4 is a fantastic game, but it will be really hard and you won't dig the story as much if you haven't played 3. Both 3 and 4 have an all star sci-fi cast and are two of the few FMV games done right. One warning about 3, some gamepads and flight sticks are a pain to calibrate without using some odd config tricks if you want analog control. Privateer has a open world design with a plot that is a heavy inspiration for Star Citizen. If you don't mind old school fun and some slowdown issues, start with 1 or 2.

Based purely on my preferences, start with 3 or Privateer.

I originally started at three and played through 4 + Prophecy., having never gone back for 1 and 2. With so many sales in the past year for the whole series, you might as well start from the beginning.