On looks alone, the bikes that do it for me are :
1974....Honda CR125, Ossa 250 Phantom, Maico MC400/440GP
1975....YZ125, CZ 250 Falta & Husky CR360 Mikkola
1976....RM125A, RM250A & Maico AW400/440
1977....RM125B, Montesa VB250 & VB360
1978....KX125A4, Honda CR250 & Husky 390AMX
1979....KX125A5, KX250A5 & Husky CR390
1980....Honda CR125RA, RM250T & YZ465G
1981....RM125X, Maico MC250 (& 490) Mega 2 & Husky CR430
1982....RM125Z, RM250Z & Husky CR500 "Silver Bullet"
1983....CR125RD, CR250RD & KX500
1984....KX125, KX250 & Honda CR500.
Usually the bikes that look the best are the most competitive as well. But exceptions to this would probably be the '82.5 Husky CR500 and the '83 KX500