While not wishing to get splinters in my chuff from sitting on the fence, surely each decade of biking has / had its own unique attractions? From the pioneers, to the development of machinery and riding kit to the present day, it is a huge and varied range of accomplishments on show. Single cylinder belt drive, twins, multis, watercooling, turbos, cafe racers, board racers, choppers, streetfighters, race replicas, trailies - look at the sheer diversity on offer.
No one decade really stands out, but if I had to choose, it'd be the 70's - I passed my test in ahem harumph er yes, the music was shite, as were the fashions, but the bikes - Jota, Guzzi Le Mans, Z1, GT750, CB750, Munch Mammoth, Van Veen OCR, DKW 2000, RE5 etc and far less traffic and nasty plod - what a time!