When looking at quality there are many aspects of a gun to consider; including but not limited to, the exterior looks and build quality, the interior build quality and materials, and performance. There are many guns that shoot extremely hard but lack in looks and interior build quality. Likewise there are guns that may be lacking in realistic looks and build quality that are built internally very well. The following is a list of the major brands of AEGs and their individual pro vs cons:
Tokyo Marui is considered to be the 'Godfather' of airsoft guns, they have many years of experience and have traditionally set the standard for quality. The internal build quality is very good, another perk to owing a Tokyo Marui is that many, of not most, aftermarket parts manufacturers build their parts to fit Tokyo Marui guns. One downside to the Tokyo Marui AEG is that the externals are mostly plastic.
Classic Army is another very popular brand of airsoft gun that has been around for a long time. Classic Army guns are more realistic because they have full metal bodies, and the stock FPS is generally harder than the Tokyo Marui counterpart. However, the Achilles heal for most Classic Army AEGs is the poor internal built quality.
G&G has made their mark in the airsoft market by providing very high quality externals and a well engineered gearbox. G&G has not been around as long as Tokyo Marui or Classic Army, but they seem to have been able to emulate all of the good characteristics from both companies and none of the bad. Externally their guns are built from the highest quality materials, and function as close to the real thing as possible. Internally G&G also uses the best quality parts available to make a reliable gearbox to withstand the high performance characteristics that G&G guns are known for.
Source Airsoft GI