Monsters of the Multiverse because of the races I didn't have and the updated statblocks. Not running new modules (still running CoS and TftYP since last year, LMoP+DoIP for a new group of people, and a Forgotten Realms homebrew adventure).
Next year very much interested in Bigby (Fitzan for me was a hit, hope they bis), and if the Book of Many Thing is what I hope it is (an "of everything" style book for DMs) also that.