Excluding the GTR's (which all look awesome) here's is what I think of the exterior (factory standard);
R31 = Ok, but a bit long in the tooth now. Factory wheels look crap.
R32 = Streamlined, yet semi aggresive. Just looks fast. Not a fan of the rear. Looks like someone in kinda was playing 'shapes' and just plonked them on there. White & black look best. Factory wheels look crap.
R33 series 1 = Yuk! Hate the front end, badges, bonnet and factory wings. Doesn't look like a sports car, more like a badly dressed sedan. Best thing is the factory illuminated rear panel. Factory wheels look crap.
R33 series 2 = Sleek with curves in the right places. More refined & smooth look. Subtle bonnet bulge & sharper headlights still retain an aggresive look though. Much better M spec wing & front bar. Orange blinkers on the front bar make it look a little dopey & outdated. Rear end just a tad bulbous (would have looked better if the boot went straight across & flat which would bring down the back slightly too). Silver & white look best. (white best suited to red tail light contrast). Factory wheels look crap.
R34 = Most aggresive stance. Looks mean as ass looks like it's slightly higher than the front. More brute than sexy. Compact. Best looking tail lights, but cheap looking (tail light) surrounds. Front lights not aggresive enough & look to 'family car' without the eye lids. Nismo front bar looks hot! Can look like a cereal box on wheels at times. Love the rear wing. All colours except yellow look great. Yellow just good. Factory wheels look semi-decent.
Interior wise (factory standard);
R31 = Outdated (but that can't be helped) Back room good.
R32 = Outdated (again, can't be helped). But good at the time. Back room crap.
R33 s1 = Can't get past that steering ugly wheel, probably nicer colour seats than the R33 s2.
R33 s2 = Nice, but a bit too simple & refined. Hasn't dated much compared to the previous models though. Back room crap.
R34 = Best. More aggresive dash. Guage mounts nice touch. But hate the poxy two tone colour seats. Seats could have been more figure hugging too. Looks like this is where the cost cutting came in. LED a/c display not as nice as R33. Back room crap.
Engine wise, every successor has gotten better.
If I had to choose between the single turbo Skyline's, then it would be the R34 GTT, then R33, R32, R31.
For the GTR's, it would be the same order. R34, R33 then R32.
This would make sense as every incarnation has been better in the majority of aspects. I don't think car manufacturers go out to make a worse next generation car.
*Note that any car can be made to look up to 50% better with some nice rims.