I actually overheard a conversation about these two singers at a restaurant a couple of days ago. Two guys were wondering which of these famous falsetto singers could hit the highest note!
So, of course, that got me to wondering (since, to my untrained ear, I cannot be certain) – which one sang higher? I am thinking Barry Gibb, but I could be wrong…
I’m not certain; they seem kind of in the same range… but if you listen to some of the harmonies on Boston songs, Brad Delp might have them both beat.
Nick Gilder 
(youtube link)
(hey, at least the two you mentioned sound like guys singing falsetto - Gilder just sounds like a woman singing, period.)
sang? SANG?
Let me assure you, Barry & Frank are both still singing.
But between Gibb and Valli, I’d say Valli.
Well, yes, but the question is : Can they hit the notes they used to hit, back in their prime? Maybe they can – I don’t know!
I have always enjoyed Robert John’s (of “The Lion Sleeps Tonight” and “Sad Eyes” fame)soulful falsetto voice, but, like you mentioned about Nick Gilder, I can see that some people could easily mistake his voice for a woman’s. Barry Gibb and Frankie Valli – not so much!
Yes, Lou has a great falsetto voice, as well.
Darren “Savage Garden” Hayes 
He also looks like a woman. His nose is a little masculine, but I’d be willing to overlook it if I was in prison.