The first roughly 16 pages of this thread were made before Avalon Hill's remake of the franchise was released.
The rest take place in the new era...So we've learned the new miniature pack simply contains some kind of vague prophecy, and no specific labels (but seems obvious comparing previously released art who several of the characters are meant to represent). So there is now an "official order" and while the Dark Company, the Maze, the Plague of Zombies, Eyes of Chaos, Halls of Durag-Dol, Revenge of the Weather Man, Running the Gauntlet, A Growl of Thunder, and the 14 Japanese quests have not been re-released yet, everything else from the "classic era" has been, now.
And still we're told while thematically and story-lore wise there is a defined order, in terms of fun or gameplay, you can play them in any order you like.
Going back in time, it seems that KK and ROTWL were designed to be an added challenge for the party of heroes that survived the original 14 quests. And the Ogre and Morcar packs in the EU were of such high difficulty, they probably were imagining the players of those quests had completed KK and/or ROTWL as well. In the US, lacking ATOH and WOM didn't matter, the difficulty of the Barbarian and Elf packs seemed to envision a powered up party of heroes who had defeated KK/ROTWL once again, though oddly it made allowances for a solo player who had never even played HQ before starting out by themselves (a position that was softened in various ways through the advice of AH in the remakes of those packs).
Doug Hopkins mentioned that ROTDM was envisioned as a pack that didn't actually need any pre-quests (despite the release of "Knightfall" as a prequel quest) and was meant to "power up a party of heroes" (from baseline). Now I discovered when we played that having a party that hadn't complete any other quests (besides Knightfall) had a really hard time against the final baddie, it still mostly worked out. The other quest books and packs seem generally to be designed for baseline parties of heroes... though ATOH gives options for playing it without the arena battles (or else theoretically unlimited arena battles between quests) and WOM gives scalable difficulty options to either play only the legacy quests and/or to start the party with some extra equipment and healing potions if they're brand new. JOD is a very hard pack but gives some scalable difficulty options in the form of the "death saves" section (three options).
SQT, PoT, and CoPD seems generally to be designed for a baseline party of heroes (the latter I thought was going to be way harder than it was) but they also allow for some artifacts to be already found, giving flexibility for having other packs (mainly the game system) to have been completed already.
The Hero Collections are designed to be usable anywhere, but KK and ROTWL call for certain heroes, so Zargon will have to make slight adjustments if those heroes have been replaced by any of the newcomers.
The "free" "digital" "online" quests all are suggested as prequels to other booklets:
"Rogar's Hall" is presented as either a "challenge of the greatest heroes" or else a "training quest" before anything else is played.
"New Beginnings" is presented as a prequel to the Game System's quest 1 (the Trial).
"Knightfall" is presented as a prelude to Rise of the Dread Moon.
"Into the Northlands" is a solo prelude (for any hero) to The Frozen Horror.
"The Forsaken Tunnels of Xor-Xel" is presented as a prelude to Kellar's Keep, despite sharing more thematic elements with ROTWL.
"Dread on the Horizon" is presented as a prelude to "Wizards of Morcar."
"Return of the Witch Queen" seems to be a post-lude or sequel to both "First Light" and "Wizards of Morcar."
"First Light" sets itself after Quest 2 of the game system and ends before, during, or maybe just after Quest 13-14.
Against the Ogre Horde originally made explicit mention of Kellar's Keep and Return of the Witch Lord happening before but these mentions were deleted in the remake version.
Return of the Witch Lord and Crypt of Perpetual Darkness both end on a cliffhanger. The blank map at the end of each booklet (in the remake editions at least) may indicate the players are expected to invent their own sequel(s).
Rise of the Dread Moon appears to take place sometime after Mage of the Mirror, and also after Return of the Witch Lord (unless the aftermath of "Knightfall" was meant to setup those events rather than be an after-event).
Kellar's Keep and Return of the Witch Lord were originally written as simultaneous threats that could be embarked upon in either order but most have decided its most logical that Kellar's Keep takes place before Return of the Witch Lord.
Spirit Queen's Torment and Prophecy of Telor give clues that they take place probably a long time after the Game System.
Crypt of Perpetual Darkness seems to portray the heroes as in the early stages of their adventures but the apocalyptic tease at the end is not mentioned in any other adventure explicitly (though its suggested that a "dark tower" might be meant to evoke WOM).
Wizards of Morcar explicitly references Against the Ogre Horde in the intro, these events having taken place earlier.
When it comes to overall difficulty, I'd defer to HispaZargon's excellent difficulty calculations...
