I actually numbered the production waves G1-G5.

G1=Optimus and Megatron Era.
G2=Combiners Era.
G3= Rodimus and Galvatron Era.
G4= Headmasters, Targetmasters, Powermasters, Pretenders Era.
G5 = Turbomasters, Predacons and whatever else was released as Hasbro's last design phase.

Officially, G2 was when they re-released some of the G1 toys with a G2 symbol and repainted most of the Combiners in new colors.
Anyone remember Bruticus in garish camouflage colors?