This is the sort of thread that exposes enthusiasts' souls.
I have two lists. One that sticks to the premise of one country per decade, and one that doesn't. For starters, I'm sticking to adult road bikes, and anything over 30 pounds is disqualified (I may fudge that for the PH50). I can barely go back to the 40's, but will reach to the 1930's for the inclusion of something Italian.
I am leaving out bicycle models created by single builders - with the exception of Rene Herse. As things move into the 50's that field begins to get rather thick in England, and beyond that, it just continues to expand. It's almost a given that a custom frame by one of these master builders is almost certainly going to be on a plateau above even the best bicycles that are mass produced.
Country per decade with no repeats:
1930's Legnano - Italy was one of the early leaders in creating racing cycles
1940's Rene Herse - arguably the most innovative and technologically advanced bicycle of the day
1950's tempted to go with a Rotrax, but there are so many great British bikes across the decades - so will go with Schwinn Paramount
1960's RIH Randonneur - Great British and French options abounded, but Holland has a great heritage that shouldn't be overlooked, and this is an elegant bicycle
1970's Raleigh Team Professional
1980's Miyata 1000
1990's Orbea Omega
And we're out of the C&V timeframe.
List 2 No limits (*) - honorable mentions
1930's - Hetchin's Brilliant (Legnano, Automoto)
1940's - Rene Herse Custom Randonneur (Bianchi Folgore, Schwinn Paramount)
1950's - Rotrax Concours (Peugeot PH50, Cinelli Super Corsa)
1960's - Peugeot PX-10 (Carlton Franco Suisse, Frejus Torino)
1970's - Raleigh Team Professional (Wilier, Fuji Ace, sooooo many others)
1980's - Colnago Master (Gitane Team Professional, Centurion Iron Man, Miyata 1000)
1990's - (not going there)