User blog comment:Hayashi H/Fall 2015 Fleet Setups/@comment-33019381-20151119091719/@comment-26486243-20151119131427

Depends on the ship you want.

http://db.kcwiki.moe/drop/map/%E3%82%B7%E3%83%A7%E3%83%BC%E3%83%88%E3%83%A9%E3%83%B3%E3%83%89%E6%B3%8A%E5%9C%B0%E6%B2%96.html

If your success rate is a lot lower in higher difficulties, might be more useful to farm lower difficulties if their droprates are similar. The one to use is the 2nd set of numbers for each ship (S-rank win drop statistics).