For NF, Crit/Mast Convoke is definitely the big one. It's solid in basically every form of gameplay, with it being very common on every boss in SoD (only Draught comes close on ST fights last I looked), pretty much every top M+ Feral was running it as of the other week, and the few high-end NF Ferals in PvP run it as well (likely with different secondary stats, mind you).
In terms of covenant-neutral legendaries, I'd recommend the following, depending on what one is looking at:
Draught of Deep Focus: Competes reasonably well in SoD on single-target boss encounters, and is top-tier in Arenas, thanks to most Ferals being Necro for that form of content. Awful in M+.
Circle of Life and Death: Solid pick for M+ content. Underwhelming in PvP or the current raid tier.
Apex Predator's Carving: Jack of all trades, master of none pick. Good, but not great in all forms of content.