OK, thanks guys. I'll get the King Arms one now and look at built-up one after I finally get to game with my rifle for the first time.

THe BEST gearbox shell is the Haynes CNC gearbox, or so I'm told. They're practically impossible to find nowadays though and quite expensive but a properly CNC gearbox should be significantly stronger than normal cast ones.

hands down, ProWin 8mm split gear box if you can find one.
have mine now for close to 7 years, damn thing is bomb proof.
nothing beats being able to work on the gears and not have to worry about the spring and vise-versa. quick spring swap is just awesome. i would say this gearbox is what ICS should have been.

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.

Partially disagree on the G&P, they're pistons aren't bad, they're just all over the place lol
Older (4 years back) G&P pistons were total crap.
Newer models from 3years ago and up are at least decent. However I've seen them last anywhere from half a season, to 3 YEARS of heavy abuse (200,000+ rounds with 380fps spring) without stripping and zero maintenance.
Same goes for their gears.
Honestly when it comes to G&P I'd say their gear shafts are the weakest link by a wide margin. I always see spur gear shafts break before pistons strip.

honestly i would have to say custom built gearbox's to your personal preference. I can't stand stock guns i always dive bomb into mine and build them to my liking.

I thank you for all your input, and I thinking of starting a super/custom gearbox using some donor parts from my SRC gen.3 pro stock gearbox. And to people talking the best shells 1) I'm not adept at airsoft internals and I don't have a gun that I can practice on, 2) I need a rifle ASAP. Thanks again(, again...)

Be very careful about going the "super/custom" route. Some gearbox shells are just not compatible with other brand components. Buying a specific brand drop in gearbox can do the same thing IE the torus not fitting into some metal bodies and having the axle spacing slightly different to accommodate their modular gearset.

It's not as easy as it sounds to just buy the absolute best that money can buy and cram it all in together, their can be some severe problems.