If the availability of competitive bikes is a criteria, 1974 would have to be the best year in MX history.
In the 250 class we had such classics as the Mag Husky, Bultaco Mk7 Pursang, Montesa VR Cappra, OSSA Phantom, Yamaha YZ250B, Honda CR250 Elsinore, KX250 Kawasaki, and KTM/Penton 250. In the "almost ready but just short on speed" we had the CZ250, Maico 250 and Suzuki TM250. Every one of these bikes was competitive and capable of winning on any given day at any given track. You name me a year when there were a dozen different competitive bikes in the one class?