In my humble opinion, the best pre '78 bikes are :
'77 RM125B (no contest)
'77 Montesa VB250 or '77 CR250 Husky
'77 CR390 Husky or Montesa VB360
I have had a ride on a nice '77 Maico AW400, but I think that my '77 CR390 is a better bike (and so did the Maico owner). I also own a VB360, which I think is a great bike, but it is totally different to ride than the Husky. The VB360 has a 4 speed gearbox and pulls from low revs like a tractor,with a strong mid-range, but doesn't seem to have much top end power. The CR390 has a six speed gearbox with a broad spread of power with a strong top end. So I think that the Husky is more suited to faster tracks, where the Monty is suited to tighter technical tracks with short straights. Jody Weisel (like him or hate him), editor of Motocross Action Magazine, rated the '77 VB250 as the best 250 motocross bike of 1977. I don't really rate the Yamaha D models that highly, but I am sure that in the right hands they are a winning bike. They probably have the best forks (along with the Maico) and front brake in the class. It is interesting that the Yamaha is never mentioned when the nine inch travel rule is talked about, yet the specifications on the brochure for the D model says they have 250mm travel at the rear. The travel limit should be ten inches anyway, as there were plenty of bikes made in 1977 with more than nine inches suspension travel.