Nightmare dungeons are substantially faster than helltides.

Helltides monsters cap out at 13 levels higher than you with a minscage.

You can easily run nightmare dungeons with monster 40-50 levels higher than you with the power creep this season.

Additionally nightmare dungeon events can be crazy. If you have a good AoE build you can insta delete waves of the event spawns. So you’re stacking dozens if not hundreds of monster kills for mobs 40+ levels above you. Helltides can’t compete.

But if you just wanna afk you can always park your character at the helltide boss.

depend on ur gears/build if u can solo nmd go for it , otherwise just spam helltide u will get a good amount of exp and remember to use profane mindcage to add 10 extra lvls to the monsters

Do events and find cursed shrines in NMD.
Use elixirs/incense , max out seasons blessings if you haven't already.

I think helltide is the fastest way + it is profitable

Helltide by far. Even if you dont use elixers. Collect Baneful Hearts and open accursed rituals.
It will attract other players with higher level what will give you the needed level ups. But be smart dont spend all your banefuls as the only person.
Because your party is over fast and the only thing you do is running from one helltide chest to the other.