Isn't the HK1 still the LARGEST airplane ever built?
Not now. Ivan will correct me but I'm sure the Antonov An-225 is currently regarded as the world's largest aircraft. I believe there have been others in between. It depends what you base it on. Wing span, length, height of the vertical tail surfaces or overall size. Some aircraft might have less wing span but be much longer & vice versa.
If some reports are to be believed, Howard Hughes was so shaken by the experience of "flying" the Spruce Goose that he never attempted it again & wouldn't allow anyone else near it. They say the only reason he attempted it on that particular day was to avoid the consequences of the Congress hearing he was involved with at the time, accused of cheating the US Government & the taxpayer out of millions of dollars. By doing that short hop he had proven his critics wrong, the aircraft had actually flown & proceedings against him were dropped.