I would personally give it to Mabui as her initiative can determine who goes first in Move 1 as well as a potentially guaranteed Immobile by Mei (due to chase). However, I have seen people put their items with more Crit, Injury, and potentially Ninjutsu and Taijutsu on either Tenten or Mei to maximize their damage potential. It will highly depend on your opponents and whether you need Mabui to use mystery first, or the damage from Tenten or Mei is more important.

~Jib