After considering numerous good value vintage groups from Japan, Italy, France....the one group that rises above them all IMO, is Shimano 105 1050. Designed to be a value leader, no compromises in performance or quality, and strategically marketed (or forced down the throats of bike manufacturers) as a complete group offering on mid level bikes.
The freehubset was basically 600 62xx, SLR brakes were better performing than their then current D-A single pivots, Uniglide 7-speed cassette with SIS, all aluminum RD. durable headset and bottom brackets. Even included their own UG chain.

I can't recall any weaknesses in the group. And it flooded the bike market. I think it was the group that brought the end to French component manufacturers, Suntour, and nearly killed off Campagnolo with their less than popular Victory and Triomphe entry level groups.

Latter 105 1055/56 would have evolutionary improvements, and continue to be a great value. But the 1050 from the late 80s was a milestone offering. The gap between it and 600 was small. The gap began to widen in the later generations.