i would say for me the one i am using hte most is kuraks dungeon XD…
whats your most frequented dungeon?
Seems that that will be Dregs for me.
I dont do dungeons often, but the one I would use for farming legendary chests would probably be sunken city. You can dive straight to the end, theres only few mobs before the final fight, so its relatively easy six legendary chests.
i been running warmaker’s quite a bit.
Kurak as well, after that, probably Warmaker. Also ran the Frost Temple a couple times, partially, because I wanted some Aesir pieces for my outfits.
For me the most frequent is like @Khaletohep said very correctly in another post, the midnight dungeon.
I do all of them, because i level up thralls all the time, like @Akomo said in another post. The one that i avoid for leveling thralls is the wine cellar, official servers are not performing so good and this dungeon can be punishing. Kurak is a safe, but a bit pointless dungeon for me, so i don’t do it a lot, just a couple times each week, that’s it.
Purges and Raids on the beta client so far, with the occasional visit to the Silve Mines, Midnight Grove and Warmaker’s Sanctuary.
I want to use the opportunity to advertise the awesomeness that the Volcano dungeon has become. Some new drops, lots of chests and very good stone and brimstone deposits. Bring a corrupted weapon and some sorcerer bags to clean the area around the boss.
Ah yes, I forgot, I farmed the coup de gras a few times by spamming that dungeon at the beginning of the chapter. Unfortunately, the coup de gras is still useless. 
@Community
It is amazing indeed. Tbh when i first visit it after the update, it crossed my mind to bring thralls in it and train them. But the npcs out and in the volcano hit so hard that’s easy to loose a thrall. When i went with high lvl thralls, then it wasn’t so beneficial for them in terms of time. The landscape is beautifull, the loot is really good, but thrall lvling is out of my check list. Fantastic revamp indeed!