From: Paul Burke

Well my (paternal) grandfather used to play the melodeon while standing on his head, but that wasn't English Folk, that was Music Hall.

I doubt we'd have to limit it to "Folk" instruments - though maybe to eras and styles. For the later part of the 20th century I guess the guitar in all its forms would be the instrument most had played at one time or another and so might qualify for many styles. 21st century? - Well it's early days yet. Late 19th Century & early 20th? The piano would have to be high up there - especially in pub culture. Concertina - all types, Melodeon and PA would also get a shout.

Any answer would obviously be an arbitrary and nonsense one but it's interesting to explore. For my money though, for it's pervasiveness, versatility and longevity it's got to be the fiddle.