I would have to stick with VFC for this one. They were solid in the past, with fantastic externals. And their newer remakes are including things like slightly better pistons/tappet plates, mosfets, and other updates.
King Arms has been coming with quick change spring guide gearboxes.
Ares have been getting worse with their EFCS picky-as-all-hell mosfets.
Don't remember if G&P have been stepping up with minor updates and mosfets, but they would probably be my second choice.