What is the best way to farm legendaries? Farm obols, helltides, whispers? I've also noticed pvp areas seem to have higher elite density so more stuff drops there too.
Yeah, elites spawn in pairs pretty reliably and in fixed locations in the PvP area, you can just leisurely cruise between those on horseback.
I usually just bloodmark myself and tour the baleful chest spawns as I go, gives you a few extra drop chances.
If you're looking for something more reliable (sometimes the game gets stingy all of the sudden and you won't see a legendary for a time), dumping 1000 obols usually results in 3-5.
For helltides, the regular chests have always been crappy for me, so I stopped bothering with them. The real deal seems to be the mystery chests (you can look this up on helltides.com)
which can drop a whole bunch of legendaries as well as an odd unique.
I don't think there's really one best way to farm, unless you want something really specific, boils down to doing whatever feels spicy so you don't burn out.
For what it's worth, about 3/4 of the uniques I have came from Nightmare Dungeons. Legion events on the other hand seem to be lousy, but I've only done a few of those.
Cosmetic items seem to drop from all kinds of different shit and it can be rather easy to overlook them on the ground,
I got one out of a baleful chest, one out of a treasure goblin, one came out of a random event, at least one as a nightmare dungeon reward. A few seem to be guaranteed from quests.