NOTE: Edited this comment to reflect current opinions and improve my phrasing, should anyone come across this

IMO there is no definitive edit, every edit has something good going for it and deserves a spotlight. It’s so subjective, there are also completely different “genres” of Hobbit edits so it’s hard to compare of them, I’d say they could be divided into two categories:

1: The trilogy/duology format is the first category. This includes the Arkenstone edit as you said, the Chris Hartwell edit which came out this year, and a great many others. I haven’t seen any of them so I wouldn’t know much about these, but in summary, they embrace PJ’s additions and just try to make them better movies. It allows for less ambitious editing, which can make things more “seamless”, but at the cost of less book accuracy or less narrative focus. I would say the best two are the previous ones I mentioned. There is also the ‘original 2 film structure’ edit which is a bit shorter than the two above I mentioned, worth checking out. If you like most of the originals but just want a bit less, these are definitive for you.

2: The second category is the 3 in 1 edits. First, to get this out of the way - edits that turn the trilogy into a single less than 3 hour movie IMO aren’t able to really be a definitive type edit simply just because they remove so much that either technical quality is scarified or narrative/character depth is scarified. Basically, while it is possible to pull off a 2.5 hour edit and they can be decent, it’s hard to compare them to longer edits which retain many more of the “good” scenes. I think most most viewers would probably prefer to have a more complete experience. Therefore, I think 3.5-4.5 hours is sweet spot, it also fits in with the LOTR extended runtimes.

Obviously my edit falls into my favorite category (the 3-in-1 four hour format), and if you’re interested you can read my thread, but in general I think that the reason this format works so well is twofold: you both get that book accuracy (Tolkien knows how to make a good story) but you also have enough runtime to play around with embellishments and keeping new stuff that might not be in the book but still “fits” the general themes/plot/world, such as additional character moments, comedy, extended scenes, more action, more drama/tension, etc. without going overboard and including an hour+ of made up stuff.

My Favorite Edits

M4 Edit (obviously bias)
Maple Edit/Bilbo Edition (tied)
Arkenstone Edition / Chris Hartwell edit / Original 2-film structure edit (These 3 are the longer category 1 edits, and it’s hard for me to rank as I’m no expert but they are all really good)
Spence Edit

Honorable mention: NameLessEditor’s version, just needed to be more technically refined but it was so ambitious in its book accuracy, and a huge inspiration.

As for LOTR, the definitive edits… are PJ’s extended edits in my opinion. Yes there’s minor things I might change, and if someone were to make a very careful and conservative edit, it could be better, but LOTR reached an amazing balance of book accuracy + creative liberties, so any edit that aims for “even more book accuracy” might be a little too much, now I could be wrong, I havent seen those edits, maybe they are relatively conservative, but as a huge Middle Earth fan, a 4 hour fan edit of the hobbit + the 3.5-4 hour official extended LOTR movies are my go to in a marathon. I will say I would trust Hal9000 with a revised LOTR edit made in 2020 for slightly more book accuracy and small fixes here and there, I think he had a project like that but it’s really old and unfinished, he should pick it back up though. But both hal’s and kerr’s old projects, as of right now, are pretty much the only LOTR edits worth watching for a full experience of the entire trilogy (I believe there are some fun ones that just follow certain characters but it misses the main story). But I wouldn’t call them definitive, because I’m sure they’d change or improve a ton of their old work if they started again today.