For me it's the old 916 race bike. Bought in primer grey without headlight, transformed into a Bostrom rep stars and stripes paintwork. Then into a standard red 916. Finally ending it's life with me as a pure race bike.
I spent somewhere ITRO of £8000 on upgrades on the bike, blueprinted and gasflowed engine, lightweight wheels, STM slipper clutch, Ohlins rear and forks, lovely race exhaust, carbon here there and everywhere.
Was a stunning machine... made a mistake selling that one.