Blueprint priority list

Accurate as of 23 June 2015.

Higher listed = higher priority

Tier 1: Highest priority tier. Fusou-class' increase in utility is much more than Tone-class', besides their Kai 2 is much stronger than the next best alternative (Ise-class Kai, at least until they in turn get their Kai 2s), both in firepower and plane slot arrangement. Also zuiun 12.
-Fusou/Yamashiro (Pick one first: Fusou for +1 firepower OR Yamashiro for extra search light)

Tier 2 Next highest priority tier. While good to have, their increase in utility is not as much as that for Fusou-class, and while their overall stats are better than Mogami-class, their plane slots aren't. They are higher than Tier 3 ships because CAVs are far likelier to be used for branching rules than Unryuu or Bismarck, and so are likelier to be used.
-Tone/Chikuma (Pick one first: Tone for +1 HP giving odd numbered max HP OR Chikuma for +1 torp)

Tier 3: Useful to have but not so urgent tier. I believe fighter power + SCAMP to be more important than a BB that can do torp cut-in (and needs 2 blueprints to be able to equip torps, at that), but your call.
-Unryuu (for quest giving Reppuu 601 + SCAMP)
-Bismarck (needs 2 blueprints) (BB with torps)

Tier 4: Nice to have but not so useful tier. Ranked in order of how good their stock equipment on remodel is. Coincidentally, the ranking also applies to the utility of the first 3.
-Choukai (best CA, but not necessarily needed. However her stock equipment is pretty good)
-Roma (higher firepower, and higher firepower increase, as compared to her sister. Has useless 381mm red gun but pretty useful secondary guns)
-Littorio (lower firepower, and lower firepower increase, as compared to her sister. Same equipment as her sister)
-Amagi (Suisei 601. Useless if you consider T99 Egusa more useful)
-Katsuragi (Ryuusei 601. Only very slightly better than Ryuusei Kai; in fact torp stat is the same)

Tier 5: So useless why do you even need a blueprint tier. Only use blueprints if you really have nothing else to use them on.
-Taigei (Only good for Type 97 931 Air Group (ASW-centric attack plane), so useless if you already got that or Tenzan 931 Air Group as ranking reward)