I've had more GBB's that I loved and worked beautifully than not. None of them were KWA or WE.
Good GBB's come from KSC, KJW and TM usually. TM are most frustrating because to make them sexually attractive you have to replace almost everything except a few little bits and pieces that other companies are too.... dumb? to make.
Case in point, the "TM" P226 which I shall rightfully call the Guarder P226 I had. Was a complete beast and probably the most realistic handgun I've ever had. But that's because damn near everything on it was replaced with Guarder parts, which in my experience have been of very good quality and decent in price. Rock solid aluminum body with nice finish, steel barrel and guide rod, 150% recoild strong, various internal parts....once assembled made one hell of a beast. The only parts actually from the TM factory were a few screws and pins and the grips which I could have bought separtely, probably to feel more realistic too. Those and the mag. So if you buy the pistol brand new, there's over $200 wasted on stuff you just replace....and wasn't worth $200 anyway (PLASTIC frame and barrel, etc.)
There is no "best GBB". They always screw you somehow. Guarder is probably the closest company to making the ideal GBB but they are just too lazy to make the final little bits.... Since they already produce 95% of the parts for most mainstream guns, why not? Other than that, you're either going to get a plastic gun that's reliable but leaves MUCH to be desired or a full metal gun that may have decent internals as well but in most cases, the metal is complete shit and the finish is pretty shitty too so theirs still awesomeness to be desired.