R32
GT-R: The base model, the lightest, but also the one with the worst tyres and suspension. The lightest of all GT-R Skylines.
GT-R N1: A version aimed for track use, equipped with different turbos and other minor equipment changes.
GT-R V-Spec: A refined model with a bit more weight, larger wheels and better suspension, and higher power output.
GT-R V-SpecII: An even more refined model with slightly wider tyres than the V-Spec.
The best one out of these.
R33
GT-R: Again the base model.
GT-R N1: A similarly track equipped one as the R33 N1.
GT-R V-Spec: A refined model featuring better suspension and a more sophisticated four wheel drive system.
The best one out of these.
GT-R V-Spec LM Limited: Just a V-Spec built to celebrate the Le Mans success of the GT-R.
R34
GT-R: Again the base model. The drivetrain of the R33 V-Spec fitted as stock.
GT-R V-Spec: A similarly refined model as the R33 V-Spec.
GT-R V-SpecII: An even more sophisticated fast car, with technical improvements here and there.
GT-R V-Spec N1: A similarly track equipped car as the R32 & R33 N1, based on the V-SpecII.
GT-R V-SpecII Nür: A special version of the V-SpecII, featuring an N1 engine with better turbos and some interior changes.
The best one out of these.
GT-R M-Spec: Something of a luxury GT-R, featuring a slightly softer suspension than the V-Spec and a leather interior. Technícally similar to the V-Spec.
GT-R M-Spec Nür: Like the V-SpecII Nür but built out of a M-Spec. The heaviest of all GT-R Skylines.
The Nismo models
GT-R S-Tune: The ultimate R32, it comes with a high revving engine and a very responsive handling. Based on the R32 GT-R V-SpecII.
400R: The ultimate R33, with loads of power in the lower revs but a bit asthmatic top end. Based on the R33 GT-R V-Spec.
GT-R R-Tune: The ultimate R34, pretty similar in style to the S-Tune but handles like the R34. Based on the R34 GT-R V-SpecII.
There you have it all.