While not necessarily the best motorcycles or the most expensive, these are some of the most influential motorcycles ever built.
Daimler Einspur 1885 Germany
More info. Possibly the first motorcycle ever built.
Harley Davidson 61EL 1936 USA

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Honda 50 1958 Japan
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Honda CB750 1969 Japan
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Bultaco Sherpa 1964 Spain
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Vespa 160 GS 1962 Italy
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Vincent HRD Series C Black Shadow 1965 UK
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One of the more famous pictures in motorcycle history. Aboard the Vincent motorcycle owned by the California sportsman, John Edgar, this final run resulted in the most famous photograph in motorcycling, the "bathing suit bike" shot taken from a speeding car on the Bonneville Salt Flats in Utah. Rollie lay flat out on the motorcycle wearing only a speedo bathing suit, shower cap and a pair of borrowed sneakers.
Interesting and Beautiful Motorcycles
This is a much more subjective category but it'd be hard to argue these aren't beautiful and interesting examples.
Indian Scout 1920 USA
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Kawasaki H1 Mach III 1970 Japan
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Matchless G45 1952 UK
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Megola Racing Model 1921 Italy
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Honda RC-166 250cc 6-Cylinder Racer 1960 Japan
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MV Agusta 350 GP 1950 Italy
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Norton Manx 1955 UK
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NSU Rennmax 1957 Germany
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Opel Motoclub 1929 Germany
Cotton 500 1937 UK
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Triumph Speed Twin 1939 UK
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Villa Gran Prix 1965
Odds and Ends
These are bikes I like for one esoteric reason or another.
DKW SS250 1939 Germany
DKW Hummel 1965 Germany
More info.
Ducati Mach 1 1964 Italy
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Ducati Paul Smart Imola 1972 Italy
FN M13 1948 Belgium
Scott Flying Squirrel TT 1929 UK
More info.
And more here.