If we are talking just for Mk2 Golfs, then clearly the best colour is bright blue metallic...not just because I own one, or even because they only made 500...but just because it looks amazing.
Otherwise it depends very much on the car itself, for instance I'm not really keen on white cars, but a Honda R-Type looks great in white. Same with yellow, most cars look bent in yellow...but a Lotus or Lambourghini really pulls it off.
Black is pretty much the only universial 'good' colour for any car, but I'd never own a black car...it only looks amazing for about 30mins after you've cleaned it!