Single tone signal generation at microwave frequency with best phase noise and spurious performance
Watch out with asking for "the best".

Suppose I have here an "Ultiwaveform RFgen extraspec. deluxe" (I just made that up, it does not exist) for sale, it will deliver the cleanest 6 GHz signal ever observed. It's yours for a mere $10 million. Do you still want "the best"?

For a clean single tone (one frequency) crystals and resonators are the way to go. In my previous workplace we used a 12.5 GHz DRO for frequency converting signals. Here's an example of such a DRO. A proper DRO will generate signal that is as clean or cleaner than the signal from a professional RF generator, and I mean the ones that cost the same as a small car.

I want to produce a single tone 6 GHz signal with a 100 MHz Sine Wave OCXO by multi stage multiplication.

Not sure what you mean there but the traditional way to upconvert 100 MHz to (for example) 6 GHz is by using a PLL. The disadvantage of this method is that the phasenoise increases with the same factor so in this case that is a factor 6 GHz / 100 MHz = 60. That's a lot and that is also assuming that the (components of the) PLL itself do not contribute additional phasenoise.