It depends.
If your computer wins the King of Sh** award then it's best to stick to CFS1. You need a lot of HD space for CFS2 but if you have an average computer it runs quite smoothly (unless of course you make a Lancaster DP with 48 exhaust effects and fly it as an AI at night. That's garunteed to make your FPS crash like a flying brick
). CFS2 has better looking models in general (with the exception of DieStaffel's models) than CFS1, but CFS2 doesn't have the shredded wheat effect when you shoot up a plane. Also, CFS1 planes don't shoot at you and hit very well from 60 miles away
CFS2 is overally better though, with it's graphics, capabilities etc.
CFS3 is pretty good too, but looking at the ground from high up and looking at coastlines just looks plain ugly
It's not that easy on your FPS either
Nevertheless a d!mn good sim and with MAW and TOW (TOW for CFS2 coming soon too possibly(definatley a campaign etc but not sure if it will be made by TOW yet. CFS3 has better vision at night though but also has certain night-fighter setbacks. OFF for CFS3 has come out already!) coming soon it's all in the sun.
Happy flying,
Bombardier