Which item appears the most useless or has the most significant side effects

Hey, now you're talking! Doraemon's gadget arsenal is like a massive game server – it's got everything. Naturally, alongside the ridiculously overpowered ones, there are also bizarre gadgets that make you go, "What even is this?"

When it comes to the "most useless" and the one with the "biggest side effects," I think we need to look at them separately; they're champions in two different leagues.

League 1: The Side Effect Hall of Fame (Whoever Uses It Regrets It)

The competition for this award is fierce, but my top contender has to be the Devil Passport (悪魔のパスポート).

What does it do? Simple and brutal: Flash this little book, and no matter what bad thing you do (like stealing money or hitting someone), the other person will instantly and unconditionally forgive you, even thinking, "It was my fault." Sounds awesome, right? It's basically a license to do whatever you want.
What's the side effect? This is the terrifying part. Its side effect isn't physical pain or a shock; it attacks your mind directly. Every time you use it to do something bad and get forgiven, an overwhelming wave of guilt and self-loathing crashes over you. Nobita just used it to get free food and take some stuff, and he ended up feeling incredibly guilty, like he was rotten to the core, practically having a mental breakdown.
From a game balance perspective: It's like an artifact in a game with the effect "Invincibility + Forced Persuasion," but the debuff is "Permanent and infinitely stacking 'Guilt Trip'." This debuff constantly drains your "san value" (sanity). Once it's gone, your "character" is ruined. This cost is simply too high for anyone normal to bear. It lets you run wild in the physical world while building an inescapable hell for you in the mental one.

Honorable Mention: Dictator Button (どくさいスイッチ) This one is terrifying too. Press it, and the person you dislike vanishes completely from the world. No one except you will remember they ever existed. Its side effect is ultimate loneliness and terror. When you make everyone disappear, leaving the whole world empty except for you, that sense of despair is worse than death.

League 2: The Most Useless Gadget, Pure Comedy Gold

This league is for gadgets that make you think, "Why wouldn't I just use my hand?" or "What is this thing even for?"

My top pick is the Automatic Booger-Picking Machine (自動ハナクソとり機).

What does it do? The name says it all... It's a huge, complex, impressive-looking machine that extends a mechanical finger to... pick your nose for you.
Why is it useless? Seriously, does this even need explaining?
1. Inefficient: By the time you haul out this clunky machine, start it up, and line it up, I could have solved the problem with my pinky finger several times over.
2. Massive Overkill: Using a 22nd-century future gadget for such a trivial task is performance art in itself. Practicality is basically zero.
3. Potential Risk: If this machine glitches and the force isn't controlled properly... well, that's a mental image I'd rather avoid.
From a game balance perspective: It's like a "novelty toy" in a game. Imagine getting a gadget in a world-shattering game with the effect "Automatically organizes your inventory," but it takes a 1-minute cast time. Would you use it? Its only purpose is probably for laughs, adding a touch of absurdity to the story.

Honorable Mention: Indoor Travel Machine (室内旅行機) This thing turns your room walls into scenery from around the world, making you feel "immersed." But by today's standards, isn't this just a really poor "panoramic VR" or "animated wallpaper"? You're still stuck in your room – you can't breathe the outside air, can't feel the local temperature. It's purely "picture tourism." For Doraemon, who has game-breaking gadgets like the Anywhere Door, this thing is basically electronic junk.

To Sum Up

So, if I had to give a final answer:

King of Biggest Side Effects: Devil Passport. It directly destroys your mind, making life worse than death. It's a forbidden artifact with a cost too high to pay.
Most Useless Comedy Champion: Automatic Booger-Picking Machine. It perfectly embodies the phrase "taking off your pants to fart," taking the concept of "utterly pointless" to its absolute extreme.