It's been a long time, and granted, this is the opinion of the current designers of the franchise (not necessarily what might have been intended back then, when the game was divided up by region with different exclusive expansions!), but Elvyler has a great list
here.
Now Avalon Hill's designer Doug Hopkins has stressed in an interview last year that he perfectly understands and even encourages players to play the quest books in the order they want to play them. He says don't worry so much about the order... that is just there to show the intended story progression but isn't required (and say nothing about the difficulty). They intend you to play the quests in order within each booklet (except Jungles of Delthrak which uses branched pathing a bit like a choose your own adventure book!). There are also optional free digital quests designed as a prelude to various expansions to give the group a chance to gain more experience with the new rules and earn some more gold or equipment to blunt the difficulty slightly, or help introduce story elements ("New Beginnings" for the Game System, also "Rogar's Hall" though most considered it too difficult as a true beginners quest, it's more of a one-off challenge; The Forsaken Tunnels of Xor Xel for Kellar's Keep although it has more in common thematically with Return of the Witch Lord, basically like a prelude to that one; and Knightfall for Rise of the Dread Moon).
Elvyler hasn't updated in awhile, but there is now "First Light" (a 10 quest "starter set" and alternate game system sold in regular retail stores) which takes place at the same time as the Game System with an alternate band of heroes (beginning shortly after the Rescue of Sir Ragnar... and ending at the time of the "awakening" of the Witch Lord).
To me it is a bit strange how people generally argue that Frozen Horror should come before Mage of the Mirror. I don't know why that is, other than that that is the order of their release in the remake era. FH is much harder... though if you disregard the solo quests (Yeti hugs against solo barbarians are game-ending unless you ignore hugs as "Into the Northlands" suggests you do, and/or add a wolf ally to protect the hero) AND take the suggestion of the new booklet to use FH mercenaries in the group quests of MOTM, who knows. The draft version of MOTM was very hard, probably harder than FH I would say (imagine fighting multiple Ogres in the solo quests for example, the final version replaced them with fimirs... so when you are fighting abominations in those early quests, it was very often originally an Ogre... and when you fight one Giant Wolf you typically originally were fighting two!). If anything though FH was slightly harder than MOTM before, and with the nerfs given to both, it's probably still about that way now.
Yes, ROTDM basically follows MOTM easily, but it doesn't have to take place immediately, it could have another quest pack in between for all we care.
A recent thought I was reading in another discussion was that maybe the original writers of FH/MOTM presumed that the people playing it had already completed KK and/or ROTWL, rather than that they were coming straight from the GS (which is what AH assumes now when writing quests, or even that the group is starting with fresh characters!). Yes, it's odd that one hero needs to be "leveled up" but difficulty wise the solos play much better with 2-3 heroes or a fully leveled up hero (which means the equipment you find is just a fancy way of giving you gold since you're bound to turn around and sell what you don't need at the first opportunity). So that sets some basic order ideas.
The idea that something follows up on CoPD so it must be last makes a certain amount of sense but as mentioned a million times, ROTWL ends on a cliffhanger that is never officially resolved as well (you might say it is partially referenced in ROTDM... mostly in "KnightFall" but even then it's not a definitive resolution).
So I'd say difficulty wise FH would be last, with MOTM ahead of it, before that perhaps the remake of Dark Company if it ever happens and Wizards of Morcar. Everything else kind of falls in the middle.
I will say that now having played through ROTDM I have some insights about it. We played KnightFall first, and I'd say that boost wasn't really needed, the heroes were pretty well fed and strong long before the end. The only snag we hit was during the final battle, where all of a sudden everyone wished they had played some other quests that gave them more artifacts (especially magical weapons). A real shock!
Ultimately I like Doug's suggestion... if you want to play a quest pack with your group, just do it. Don't wait around until you've played dozens or hundreds of hours of other adventures you want to play less, because you might never get a chance to play the thing you wanted with those people! If a pack isn't working, just jump to the one you want. Forcing people to sit through ages of stuff they're not interested in to get to the thing they really wanted could be frustrating. Yes, FH/MOTM are a bit broken, but the rest of the packs are designed so that as long as you've played the GS (and sometimes even if you haven't...) you can still enjoy it and not expect TPK's will frustrate you to the point of quitting the pack (or the game) entirely. And yes, there's a lot to be said about a newcomer to the table with a basic character getting gear/gold shared by the "veteran" heroes. Zargon can tailor the pack to the group, but I agree that it's nice if it's already in a fairly good serviceable condition as not everyone sits down to play HeroQuest with the idea that a lot of revision and "homework" or prep is needed beforehand. Save that for when you've played everything you wanted (or can afford) and want to rig up something completely new.
We've found that saving the character sheets (and passing them on to a new player, unless they really want to start from scratch) keeps things going as new players come and go. After awhile it could become intimidating to start from scratch when everyone else is already maxed out. Why won't they share any spare gold/gear? Why "haze" the new player by forcing them to play solo quests that are so hard they can only win by sheer luck? A lot depends upon the temperament of the player, but I think if you're going to bring in somebody brand new, it's best to ease them into it, unless they really are a glutton for punishment and determined to survive no matter what.