Honestly this is a tough choice for me to make a decision on but I would have to default to the C5 ZO6
03 Cobras are nice but the shifter and seating position is less desired IMO. I do not like reaching to the firewall to shift, it feels unsettled. The seats are ok in it as well not real supportive IMO. I do like the FI from the factory and the ability to make power easily but it just isn't enough to get me to buy one.
GTO- I just don't feel all that good about them at all I have not driven one but I just think they look blah!
C6 - G/F's father has one and I drove it and it was NICE! But if you are not getting the C6 ZO6 then don't bother.
C5 ZO6 - Love the looks, handling, seating, and shifter position. It is a well refined car that is light and can be made fast and look good doing it. I will have one hopefully in a year or two unless something arises or the C6 ZO6 is cheap enough by then.