I myself have owned pretty much every brand out there over the years and my favorites are easily G&G and CYMA. Yes, personal preference.
If you don't want to be disapointed later down the road, don't buy a KWA. Porkchop was not 100% correct in his statements about them. They have SEVERAL proprietary parts that you usually have to end up buy from KWA's 'Pro Shop' and a lot of times they're out of stock. Even the horrid 2GX gearbox is somewhat proprietary. KWA also uses 2 piece gears on the spur and sector and maybe the bevel gear as well. They also use a metal air nozle that doesn't seal well at all. Lastly, KWA M4's are rather heavy in comparison to some other makers.
CYMA AK's are simply beastly. They do have some downfalls in the gearbox, but nothing that seems to effect them much. They have decent gears, but I doubt they're steel. That term gets thrown around too much anyway by people that have no clue what they're saying. Just because it's metal doesn't make it steel, and even though a magnet might stick to it still doesn't make it steel. Anyways, back to the internals. They do sport metal bushings. They have the average mediocre white polycarb piston and ported piston head. It should be noted that all AK's are stock with a one piece hopup unit. Also, CYMA has recently started making their own specific bucking that is WAY more effective and has much more longevity than the stupidly over priced KWA 2G bucking. No, they're not a Cadilac on the inside, but they are a REALLY solid gun externaly. I have yet to see a CYMA AK take a dive on the D9 team and there are over 10 of them in use with us. For the money you save on the initial gun, you could afford to make an absolute BEAST with the leftovers. I know I could build a gun with the same $300 you might spend on a KWA M4 out of a CYMA AK that would make the KWA go crying home to it's mommy. I know because I've done it....3 times. Keep in mind though, the CYMA AK's too are fairly heavy. Also most shoot too hot even for outdoor play, so if you go with a CYMA AK you'll probably be looking into downgrading the spring immediately.
If you've got to have an M4 and you want one for CQB, I hear tons of good reports on the G&G raider. The cool thing about the G&G pneumatic blowback's is that I have yet to find out without a full cylinder in the gearbox which would allow you to drop in a barrel easily up to 550mm without any ill effects from 'suck back'. G&G doesn't use two piece gears like KWA and they are pretty good quality, not the best, but good. They're black polycarb pistons are great and every bit as long lasting as KWA's pistons. G&G's also come with an excellent ported piston head that give some of the best airseal I've seen out of stock guns. G&G also sports an airseal air nozle. The Raider will also have a one piece hopup unit. G&G radiuses the front of their geaboxes where the front of the cylinder sets which does wonders to prevent cracking, rather than thinking like the morons at KWA and saying 'well we gotta make it BIGGER to be sronger!'. G&G guns use 8mm bushings I do believe and I have no idea what the stock bushings are made of but it's not the greatest stuff. I've personally never seen one break, but it wouldn't suprise me if it did. Their wiring is VERY small. G&G brags on their wiring, but personally it worries me being such a small gauge. That being said though, that wiring cunducts extremely well in comparison to other stock guns' wiring of a larger gauge. With G&G's nylon fiber body, your gun will have more strength than most guns with metal bodies while keeping it much lighter. In one of my youtube videos of OP Bright Rain VI, you can watch me running across open ground with my G&G M4 and taking a spill right into the mud and sliding in it all right on top of my nylon fiber body M4. I pick it up immediately and no later than a minute into the video (with no breaks) you see the gun is fine and still operating properly. Even after I got home I went over that gun everywhere to try to find breaks or fractures and didn't find a single one.
One brand people keep leaving out on here is VFC. VFC is 'up there' as far as overall quality. I'd have to say they're like getting a KWA that is lighter and has MUCH MUCH MUCH better externals. They do have some weak points in their internals. I just got a VFC SCAR L from SoNorUs that he had to replace the piston in. So far though, that has been it's only failure. SoNorUs said he did a lot of research on the SCAR when he had it and his research showed that the pistons were indeed the first and most likely part to fail on a VFC. With VFC though, you'll probably be blowing all of your money on just the gun. That's probably the biggest downfall I can think of with them seeing as I have yet to crack open the gearbox on mine to judge the internal quality.
