Whoa, someone actually voted for G&G?
I've never seen a G&G gun that didn't have major problems. Ever.
Marui's are the most reliable, but lose points for their lack of field-worthiness. After one day of hard use, screws come loose, and they creak and wobble.
CA's, if left alone, are fairly reliable. (I have two and they have been excellent through several games and many thousand rounds each). Some people have issues with CA guns, but I never have, nor have I known anyone who did, until they f*cked with by trying to upgrade themselves.
Building it yourself is probably the best way to go, but it's also the most expensive, and can be a bad idea unless you know exactly WHAT parts to build it out of, and HOW to build it correctly.
Ganef's M4 is one of the nicest AEGs out there, and it was built from the ground up. However, he is very mechanically inclined, and had some solid guidance regarding which parts to use.