I've almost got 2 more mk5 furnaces, and at least a dozen toons who need them. I think my best options are Maul, Boba, TFP, Rex, and JKA.
I feel Boba might be the most useful, and TFP is great but I enjoy Maul more. Also, my LS squads are significantly weaker than my DS squads and I really could use some extra firepower for LS stage 9 hard nodes. Rex is definitely a better toon than JKA in the long run but I've spent a lot more time developing little Ani. It would take a little longer for me to get Rex to the point where I can consistently throw him into the mix. The same can be said of TFP, as I've spent much less time on him than Maul or Boba and he's only 4* at the moment.
Anyway, in what order would you distribute the furnaces between these 5 characters?