Rules: must have been or be realistically available, not "factory race team specials" ie Joe Blogs or you or me could have bought one from the manufacturer or a dealership.
Open class - Sonic 500 (2nd choice but I think I am get into factory racing specials area ,any Clews CCM BSA derivative)
250cc/450cc- 1974 Ossa Phantom Just visually sublime. (2nd choice Big bore kitted Honda 73/74 XL350, only because been there, done that love hate relationship.)
125cc - Suzuki TM125 Easy to maintain and ride, reliable low stress fun. (2nd choice Yamaha MX125 - as for the little Zook!