Let’s put the theory to the test. Editions are not listed chronologically but by how dead the PCs on the cover will be once the next round of combat starts. The deader the character, the better the game. I’m sure anyone reading this knows what the artwork associated with each edition looks like, but if you need a refresher, here’s a link to most of them.
Companion D&D: Holy shit is this dude toast. That giant sword has him feeling confident, but that dragon is about to wreck him straight up. Our hero made it at least to level 15, though, so he can meet death with his head held high knowing they will sing songs in Sto’Vo’Kor and the women shall weep and his vassals shall raise the banners and avenge him and all that stuff you're thinking when a green dragon is about to poison you, swoop down and claw/claw/bite you, and then toss the rattling, leaky metal can of bone fragments that used to be you off of a cliff.
Mentzer Basic D&D: That red dragon looks positively gleeful, wiggling his claws in anticipation of the bounteous meal he is about to receive. Praise Tiamat.
D&D 5e: Pretty sure the character leaping at the giant is about to get the treatment Saturn’s son got in the Goya painting (or Reubens, if you prefer).
Rules Cyclopedia: I see a distinct lack of confidence on the face of that horse. That's the face of a horse that has just shit himself for the last time. He’s knee-deep in water, he’s got 250 lbs of armored warrior on his back, and that creature doesn’t look like it’s going to run out of neck anytime soon.
AD&D 1e (wizard cover): This wizard seems to be fucking shit up, but it’s just him out there and he’s surrounded by imps or mephits or whatever. He’s got that look of grim determination, like he’s buying time so his party can escape and he’s come to terms with his impending demise.
Holmes D&D: Tough to say. I have a feeling these guys might make it. The fighter has his bow drawn, wizard’s got a spell cooking, they’re on the attack. But still, two dudes against a red dragon. Tough call.
Moldvay/Cook Basic D&D: Another one that is up in the air. That fighter with the spear doesn’t inspire much confidence.
Expert D&D: Depends. Who is this wizard? Is he making that face because he’s scrying like, “oh shit, my buddies are about to eat it”? Or is he all like “muahaha my plans are coming together”? This is the type of mystery and sense that anything can happen that makes B/X so great! But I’m ranking editions according to the criteria above, so I have to put this one near the bottom.
OD&D: This PC is decidedly not dead—unless he’s about to fall off of that horse.
Master D&D: Guy’s riding a dragon. He is the master. He’s mastered stuff. Stuff like dragons. He lives.
AD&D 1e (Trampier cover): You can tell these guys are true dungeon badasses. Their shit is 100% together. They’re making a map and everything. You just know they’ve got 10’ poles, they’re checking for traps, they’re only fighting when they absolutely have to. They are utterly competent, but not flaunting it like the 2e guys below.
AD&D 2e: This is just a hi-fi version of the dude from the OD&D cover, helm wing’d, upon his steed, but also not about to fall off of it.
AD&D 2.5e (black cover): Even less dead than his 2e counterpart. There are about to be some dead orcs in that room. He is almost totally unarmored because those are +5 bracers of armor he’s wearing. I mean, must be, right? He might even be yelling, “eighteen double oh!”
D&D 4e: That dragonborn is pissed, and that wizard is just annoyed that she even has to bother blowing up another gang of 1 HP “minions.” And lo, the tradition of frownyfaced heroes is born, the era of the frownyface hero reaching its apotheosis in the petulant countenances of anyone who has ever held a weapon or cast a spell in a Pathfinder book.
Immortals D&D: The dragon (presumably the one from the Holmes box, long after eating those two PCs) sits pensively on a cliffside on a distant planet, all like, “nope, not fucking with the guy in a loincloth with Psylocke’s psychic knife, not at all.” That character is going to live—he is immortal, he has inside him blood of kings.
Exempt: D&D 3e and 3.5e, which didn’t have people on the cover of the Player’s Handbook.
So there you have it, a total waste of time.