Kulikov is the easy pick. Most last year believed he was signed to play with Buff. With Enstrom gone he's the defecto #2.
Obviously neither Chairot or Morrow deserve that distinction, although it's possible either ends up paired with Buff & could be misconstrued as the #2 for this reason only. That same logic however could technically make either the #1 if you consider Buff the #1 defensemen on the team.
Niku is certainly a possibility & will definitely get a look see should an injury develop, but a healthy roster on opening day surely has him on the Moose. Should a trade occur my thinking would change radically. Niku certainly is talented enough to make the roster, but barring a trade where's the room?
It appears the Jets will go 13F-8D-2G to start the year. (Note these aren't suggested lines & pairings...please no comments)
*Connor*-Scheifele-Wheeler
Ehlers-Little-Laine
Copp-Lowry-Tanev
Perreault-*Roslovic*-Dano/Petan
Morrissey-Trouba
Kuliokov-Byfuglien
Morrow-Myers
Chiarot-*Poolman*
Hellebuyck
Brossiat
Those with asterisks (*) are waiver exempt, but each are basically locks to make the line-up.
So there's no room for Niku, unless it's on the bench & demoting Poolman, but it would be bizarre logic to demote a RD & go with 5 LD's.
Best thing for the organization (& Niku) is to keep him on the Moose anyways. The Jets have screwed around enough defensemen by healthy scratching them, Poolman being the current example. Tucker may be happy making NHL money now, but his development has being hampered everyday he sits. Think what you want of Postma, but I feel he'd have become an NHL regular had he been given the chance to play.
Nothing beats game experience, at any level in any sport.