All of them and none of them at the same time.
It's a shit answer, I know.
They're all equal in terms of what you get in the box: a gun that is for the most part in working order and that only requires a battery and some bbs.
Diving deeper into how each manufacturer does things, I found G&Gs to be lagging behind in terms of technological solutions, on the basic CM16 models the hop unit is the old style with three gears which is known for undwinding itself with use and it's not precise in adjustment, their gearboxes are not the strongest out there, they don't have QC spring guides (it's not mandatory but it's a nice touch should you ever get into teching) and the bushings they use are bad (brass). Also G&G barrels are abysmal, very low quality pressed tinfoil that gets scratched if you look at it funny.
Specnas (Core series) on the other hand sport 8mm steel bushings, QC spring guide, the absolutely fantastic ZCI rotary plastic hop unit and decent quality (for the price) 6.03 brass inner barrels. They also come at a slightly lower price point than G&Gs which opens up more room for upgrades should you decide to.
Cymas have made a huge leap in quality recently, I remember Cymas being rather bad when I started playing some 12 years ago, they would struggle to survive three skirmishes without breaking something. Now though, they punch well above their weight when you consider how little they cost. AKs in particular seem to be the n.1 alternative if you don't want to spend E&L/LCT money. I had a go with a Cyma M14 socom not long ago and honestly it didn't feel like a €150 gun at all.
ASG does mostly distribution (so rebrands) with one exception, the scorpion evo, which is the only gun they make in house (to all the Bren fanboys, it's made by King Arms).
What style of gun would you like to have?