I vote Halloween.
I read the Long Halloween first (and I believe it came out first). Many of Loeb's subsequent Batman stories sort of followed the same mystery template where a confused Batman is rotating from villain to villain as he slowly unravels the truth. It was more magical for me the first time around so I put Halloween at the top. Hush also felt like more of a platform to showcase Jim Lee's art than tell an intricate story. You can also see the ending coming a mile away, whereas the last few pages of Halloween genuinely surprised me.