Depending on which Cobra you want to model:
1/24 427 Cobra- Go with the Revell/Monogram kits. The SC and street versions have been released several times. Good rendition of the large tube/coil spring chassis and the 427 engine in these kits is one of the best ever. There is also a Fujimi 427 but it pales in comparison to the Revell/Monogram with poor chassis and engine detail.
1/25 289 or 260 Cobra AMT is a pretty good one considering it's age. Looking at photos of the early Cobras they may have used the 260 version as the source for this. Pretty good leaf spring chassis considering it still uses metal axles. this has been released a number of times by AMT. Various versions of the AMT kit have been released by other companies and some have used the 289 chassis with a 427 body.
1/24 Daytona Coupe I think the best is Historic Racing Miniatures resin kit.It has beautifully done, well detailed parts. Pricey at $185 but it's gorgeous and the only one with an engine.
Gunze Sangyo made a curbside but they are difficult to find. Scale Motorsports also has a resin curbside kit that is supposed to be good. It is cheaper, at $135, but has no engine.
1/43 Monogram made a 289 Cobra roadster, part of their Ultimates series that is also quite nice. These pop up on the bay every once in awhile.
There was a 1/16 MPC 427 Cobra but I think the detail is lacking and it seemed rather heavy-handed.
Hope this helps.