Classic Legoland (1968 - 1975). No minifigs*, buildings have walls on all sides. * I'll make an exception for sets such as 362
which do contain old-style minifigs but they cannot enter the building or vehicle.
Classic Space (1978 - 1988). I'm still getting used to astronauts in other colors than white and red.
Fabuland (1979 - 1989). Preferably with 'old style' eyes.