Humm tricky question..
I guess Killua and Alluka flashback would be awesome, specially Alluka's.
But a Netero's flashback would be incredible too, with him heading the Zodiacs and everything xD
I know this is kinda obvious but I would love to see beyond netero's flashback.
I don't want any characters flashback. Flashbacks aren't really Togashi's style. But if I had to, I'd like to see the flash back of Kite.
LEORIO!! Well just because, I think it would be nice to peer into his to past, Learn more about Leorio.
I'd like to see a flashback of Netero during his time with the Seirin Group.
I'd like flashbacks of the spiders if I had to choose. Preferably all of the original ones and Chrollo.
I'm going to have say absolutely no to Hisoka. I actually like that there is no allusion to his past at all. It adds to his air of mystery and fickleness.
I would like to understand what happened to Kurapika in all this time.
Kurapika, Zoldyck family and Genei Ryodan...
I'm not much for flashbacks in general. Most series screw them up, not that I'm saying HxH will. The only series I can say that has flashbacks that I've thoroughly enjoyed as much as its normal arcs is probably One Piece.
We technically got a bit of a Leorio flashback in the 1999 series during the Hunter Exam...but I honestly found it boring.
Ging flashback just so that we get to see some of the stuff he's done as a hunter. Other than him I'd like to see a Chrollo flashback.