[Azathoth]

If you want a complete gearbox, None of them are "the best". Each brand of complete box has a set of problems.

Systema: Bad piston, Price
G&P: bad piston,
King arms: bad gearbox shell, bad shimming, bad wiring
Deep fire: Overpriced, bad piston, bad shimming
Jing Gong: Like playing the lotto, usually awful grease
KWA: Bad cylinder head, shimming hit/miss

If you want Gearbox shells:
8mm Prowin that has been discontinued for god knows how long
Haynes CNC - impossible to find you're looking at 400 dollars

I like the KWA gearbox shell simply because the style of spring guide and how it sits in the box, and they are not reinforced in the wrong places.

"CNC" gearboxes are hit and miss sometimes you'll get one that are decent and other times the axle placement on the shell is going to be off, or some other deficiency. I scratch build my gearboxes with parts that I want to experiment with. For a novice drop in solution buy any of the above complete boxes and be prepared for the 'problem' part in them.