This is an idea that's been knocking around in my head for a while, but playing some emulated games with Steven (my 8 year old) this evening* reminded me about it.
Video games used to have difficulty levels that you could choose before you started the game. I'm sure there are still a few games that use them, but one reason I don't play a lot of video games anymore is that they seem to be designed to either give you "an experience" or else they want you to subscribe/pay lots of microtransactions, so either they are too easy (experience or subscription) or too hard (microtransactions), with no choice. But back in the day, we had this.
So, here are my very subjective and probably wrong estimations of which version of D&D is at which difficulty level. This assumes a few things. One, it's difficulty for the players to play the game, not for the DM to run the game. Two, it assumes you're running things more or less by the book, at least as far as assumptions for things like encounters, healing, goals of play, and the like are concerned. If you play 4E in an "old school style" then that's outside of what I'm talking about here. I'm considering a group that plays 4E (or whatever edition) as the designers intended it to be played. Three, let's leave supplements out of the equation for now, they just complicate things. So no Skills & Powers, no Greyhawk/Blackmoor, no Unearthed Arcana, no Tasha's Cauldron of Everything. Just the core rule books.
And I'll reiterate -- this is just my feeling about it. Feel free to tell me how wrong I am down in the comments. But the next time you start up a campaign, consider selecting the rule set that fits the challenge level you wish to give the players.
I'm Too Young to Die (Very Easy Mode)
4th Edition D&D This is about as easy as it gets for the players. It's designed so that you would have to go out of your way to create a "suboptimal" character. The play assumptions are two to three easy fights then a tougher but still winnable "boss" fight as an adventure. Magic items are fairly easy to acquire, and you're not expected to have to do much more than ride the railroad from set piece battle to set piece battle, with a few "skill challenges" here and there to spice things up.
5th Edition D&D A bit more challenging than 4E, but still a lot easier than most other editions. It's possible to create a suboptimal character, but the rules tend to be a bit more forgiving with character creation. Advancement is very fast at low levels. Healing is ridiculously easy. And again, the adventures seem to be mostly an assumption of a few easy fights leading up to the boss battle. If players just go along and make sure to rest often, and the DM only places recommended encounter difficulties, it's not too hard at all.
Hey, Not Too Rough (Easy Mode)
2nd Edition AD&D The rules and systems for play, including character creation and character advancement, can lead to challenges for the players. You might get stuck with a suboptimal character through dice rolls as much as through character choice. But, the big mitigating factor of this edition is the design goal that players play "heroes" and go on epic narrative adventures. So while death is very much possible from the way the rules are written, the DM advice suggests that this be mulliganed or nerfed to serve the ends of the story.
Hurt Me Plenty (Normal Mode)
BX or BECMI D&D I'm lumping these two together because while BECMI incorporates a lot more complexity of play at the high levels (not to mention Immortals level play being a completely different and more challenging game), at the earliest levels, play is pretty much the same in them. Character creation by the book can be a challenge (roll 3d6 down the line), but ability score bonuses are more generous than in the AD&D line. There aren't many choices to make at character creation, either. Adventure design assumptions are that encounters are not balanced, and it's up to the players to know when to push on for more and when to quit. But there are also rules that make treasure pretty generous, which speeds up advancement if the characters do survive.
3rd Edition D&D This edition has a lot of the design assumptions of the later editions. Character creation is generous with abilities and ways to optimize the character, but the complexity of the "exception-based rules" design, with all the skill points and feat choices and whatnot make it more of a burden to play than other editions. The adventure design assumptions are not quite so forgiving, but still, healing is fairly easy to get, magic items are easily purchased, and it's pretty easy to get around the "save or die" type effects. If the rules weren't so complex and fiddly, this would be in an easier tier.
Ultra-Violence (Hard Mode)
Original D&D It all started here, and it wasn't easy! Characters were randomly generated and didn't have a lot of "powers" to rely on. Monster encounters can easily be with overpowering odds. There's an assumption of thinking your way through encounters, rather than just hacking and slashing. You're dead at 0 hit points, and healing is not easy to come by. The incompleteness of the rules (remember, this is assuming the base rules only, not the supplements) may also up the difficulty a bit, as the DM will need to make a lot of guesses as to what's an appropriate challenge, and players will have to have their wits about them to survive.
1st Edition AD&D This edition has a good mix of difficulty in character optimization (it's got generous die rolling for ability scores but stingy bonuses for high scores, race/class combo restrictions, ability score restrictions, level caps for demi-humans, etc.) and difficulty in adventure assumptions. Monsters are challenging. Tricks, traps, and whatnot are expected, and can really mess you up. Sure, there are lots of opportunities to find powerful magic items, but the most powerful have serious drawbacks. And the level of detail in the rules give the DM all sorts of ways to make things difficult or more challenging for the players.
Nightmare (Extra Hard Mode)
Holmes D&D Rolling 3d6 down the line for stats and rolling your hit points randomly and you can only go up to 3rd level, but the book expects you might run into all sorts of dragons, vampires, purple worms, and the like? Yeah, this is the most challenging version if you play it straight.
*We have a Super Console X, an Android TV box with EmuElec, Retroarch, and about 30 systems emulated, with thousands of games. Tonight, we played some Twisted Metal on PS1 and Gauntlet 4 Quest Mode on Sega Genesis.