There NEVER was a 55mm f/1.2 Nikkor made in Ai-S mount. Never. The original 55mm f/1.2 was introduced in 1965. At the time, the 55mm f/1.2 S.C. Nikkor was the fastest Nikkor prime lens. Focusing on this lens is ultra-critical, and at 0.6 meters, there is around six millimeters of depth of field wide open. Admittedly, the 55mm f/1.2 was not one of the absolute best lenses, but somebody here is complaining that a world-class "speed lens" designed almost fifty years ago, and which hasn't been produced since 1977, wasn't a very good lens? (Regardless....that same lens will mount and shoot on a D40 or D60,without modification.)
In July of 1978, the 55mm f/1.2 lens was replaced by the economy model 50mm f/1.2 in the then-new Ai-S mount. AND of course, the 1977 introduction of the 58mm f/1.2 Noct~NIKKOR, the lens that now sells for $3,500 to $3,800 on e-Bay because it's one of the world's best low-light and astrophotography lenses. Yeah, Nikon lenses suck; 10- to 40-year old manual focus lenses people line up to pay $3.5k for? They must suck!
Nikkor 55mm f/1.2 Standard Lenses - Version History - Part III
As far as Sigma versus Tokina...it's a lens by lens decision. Sigma has made a few truly fine lenses; their 15-30 and 10-20 wide zooms have been very highly regarded, as have their 70,105,150,and 180 APO macro lenses. Tokina has long had good build quality, but in my opinion, has put out about as many turkeys as good models. Their 100/2.8 macro tests out very well. Again, the 3rd party lenses have to be judged on a model by model basis,not as a brand in total. Tamron's 90mm f/2.8 macro has had a sort of cult-like following since the 1970's; I own one,and it is a fine macro lens! Tamron's 24-135 is a wide-to-tele zoom lens that has reputation as a strong performer. Tamron's 200-500mm has a good reputation too. The images their 180 macro puts out look stellar to me.
Same with Canon versus Nikon; both companies have a few dogs in their lineups.