My own least favorite edition is 3.x (and PF—which sucks because I like the company, the setting, the characters) but I'd rather play 0e, B/X, 1e, 2e, RC, or 5e over 4E. I was playing B/X (with some of the guys who would later go on to make Mörk Borg) during the tail end of 4E, before 5e came out (which is when I started my own group).

There's a lot of things that's neat and good with 4E. Clarity, balanced classes, interesting–albeit–long-winded fights, neat art and settings, retro vibes.

Some of the problems only apply to the early days of 4E and was addressed as the game evolved:

• A bad initial release and marketing campaign that was seen as disparaging older fans while simultaneously begging "the game will remain the same"
• A fight against other editions and a push that "this is the game now"
• A switch to digital that was plagued with problems due to a horrible tragedy
• Broken monster math
• All classes felt overly similar in their play patterns. Not 100% identical but too similar
• Abilities that felt as if they were spells, like a fighter ability that makes all enemies take a step towards her
• Weirdly flavored / templated abilites that felt like "hitting the play button on a canned action" as opposed to actually doing it

Other D&D (including late 4E): "I have this sword. I'm gonna chop 'em with it!"
Early 4E: "I am going to use this Reaping Strike ability."

To me, that can feel cold and distant. The fact that the powers came on cards that you'd flip as you used them was actually good overall, but it exacerbated this problem greatly. I get a lot of pushback on this from people who had gotten over it and could easily "translate" the abilities into game action, like "I strike them reapingly" or however that would sound, IDK, because I never learned to do it. I can write novels and poems and lyrics but I can't strike reapingly. 🤷🏻‍♀️

4E Essentials (the later edition) get a lot of hate from fans of the original release but it has classes that I actually want to play.

Another issue I get pushback on is that some 4E fans love the "mythicness" of the martial power source. Samson with the jawbone, Flex Mentallo... And that's fine, I'm happy for them. I personally prefer where spells are explicitly marked as spells, as 5E's "Hunter's Mark" ability is or the eldritch knight's "Shield" spell, and magic items are similarly special. Like, we all love Luke Skywalker but he didn't solely rely on the martial power source. He also had ki (or whatever the midichlorian heck is going on).

And some other problems persisted throughout the game's entire run:

• A skill challenge system that was poorly explained (some people were like "always say you're in a skill challenge", others were like "no, just roleplay out a scene and as they are doing checks in the scene, mark progress")
• A skill challenge system that has utterly broken math (throughout all three revisions, including the attempt in the Essentials Rules Compendium). The 5e version, "group checks", also has problems, but fewer.
• Unnatural language. 5e had a push towards using natural language, or language that looked like natural language, over the arcane shorthand. This is a double-edged sword since it also helped with the clarity of 4E. 5.24 is back-pedaling a bit here, with capitalizing rules terms and conditions. Not my preference, I liked the natlang approach. Helps with rules-hacking and glogging and mashing together stuff from various editions the way I like it.
• Overwhelming in play with too many conditions and modifiers to juggle
• Not suited for the type of sandbox play I like. There are 30 levels and huge steps between the levels. So most people built encounters just-in-time, just to fit the party's current level and capabilities (and when you do that, the DM has a lot of responsibility for the outcome of those fights). I prefer making a world with graduated difficulty and set the player characters free in that world; if it's too easy they won't get much XP so they'll see harder pastures, and if it's too hard they die, so they'll try to find a lagom horizon on their own. And, they can freely use resources like town militias, religious warriors, pit faction-v-faction etc (we just came off a long "PCs+lizardfolk vs human slavers" skirmish campaign in our game)
• Killing the OGL!