So, I’ll get this started:

Setup: a dungeon corridor leading to an arch way into a room, on the other side of the room is another archway.

As soon as a player enters the room, the archways hidden portcullises drop down. Then darts begin pumping out of the walls at a furious pace. Only one person at a time can try to reverse the portcullis mechanism, or, if they look around, there is a secret button that disables the trap for 1 hour.

The simplest and funniest trap I use is a simple trip wire. Combine it with something else and a laugh will ensue. I usually have them in Kobold caves. You trip in the lue, now you can be smelled a mile a way. You trip in front of the sleeping area, loud noises, you wake the entire cave, your prone, and you've got disadvantage to initiative. Those are less lethal options, but you could easily make it lethal.

Pit trap with a gelatinous cube at the bottom.

I like traps which make sense - which usually means pretty simple ones. 

Realistic traps are security devices, which means they're either alarms, or barriers. Barriers don't let you through ( or stop you, if you try ) unless you know something ( where the secret disarm lever is ) are something ( light enough not to set off the pit trap ) or have something ( maybe the disable mechanism needs a key ).

Edit: I'm adding a 3rd kind, after some thought: tags, which highlight you to security, see paragraph below.

Because barriers are meant to stop you from accessing  an area, they usually should be lethal to the common person - which often isn't going to kill an adventurer, but they should be nasty, even so. Certain less bloodthirsty organizations/individuals could use a trap meant to incapacitate, or imprison, an intruder, coupled with an alarm to bring in security. I do like the idea SladeTracey posted above of a trap which tags you, and makes you easy for "security" to track and/or pick you up. That kind of reminds me of the "exploding tear gas and blue dye fake bundle of money" that I've heard used against would-be bank robbers, and which detonate once the robbers leave the bank, leaving them incapacitated and indelibly dyed blue.

I really dislike some of the more impractical and Rube Goldberg traps you see some people coming up with, because there is almost 0% chance that anyone would build such a thing. They're on a par with out of context meaningless abstract puzzle room, IMHO.

Quote from Vedexent >>

I like traps which make sense - which usually means pretty simple ones. 

Realistic traps are security devices, which means they're either alarms, or barriers. Barriers don't let you through ( or stop you, if you try ) unless you know something ( where the secret disarm lever is ) are something ( light enough not to set off the pit trap ) or have something ( maybe the disable mechanism needs a key ).

Edit: I'm adding a 3rd kind, after some thought: tags, which highlight you to security, see paragraph below.

Because barriers are meant to stop you from accessing  an area, they usually should be lethal to the common person - which often isn't going to kill an adventurer, but they should be nasty, even so. Certain less bloodthirsty organizations/individuals could use a trap meant to incapacitate, or imprison, an intruder, coupled with an alarm to bring in security. I do like the idea SladeTracey posted above of a trap which tags you, and makes you easy for "security" to track and/or pick you up. That kind of reminds me of the "exploding tear gas and blue dye fake bundle of money" that I've heard used against would-be bank robbers, and which detonate once the robbers leave the bank, leaving them incapacitated and indelibly dyed blue.

I really dislike some of the more impractical and Rube Goldberg traps you see some people coming up with, because there is almost 0% chance that anyone would build such a thing. They're on a par with out of context meaningless abstract puzzle room, IMHO.

 Actually, think about these situations: someone who wants to test the adventurers, or a mad mage who wants to study adventurers.

I have thought about them - I just find them hopeless trite cliches and completely improbable.

I don't recall anyone hiring "private security firms" by subjecting them to live fire exercises, first. If their reputation doesn't support hiring them, then maybe send them on some small, low priority jobs first, you don't try and kill them off with Rube Goldberg traps, and if they survive ( and don't henceforth try and kill you back ), offer them a job.

In my opinion, DMs don't add that kind of thing because it makes any sense from a realistic narrative point-of-view. They add them as a "let's see if I can outsmart the Party" thing, or they do it because it is a stereotype, and they think they are supposed to do it.

Not to see if they can be hired, but to test if they can handle X. Or if you are just strait up insane and evil, you don’t have to be logical

And the real world is just stuffed full of evil human test-to-destruction labs?

Don't get me wrong - when it comes to your game, you might be running a "pulp fiction stereotype" style campaign ( and there is nothing wrong with that ) - in which case your local analog of Dr. Evil can do whatever nonsensical acts they want ( for no other reason than "they're evil").  That's a pulp fiction trope, after all.

It's your game - no one gets to tell you what you should have in it ( not even me ). I'm just giving you my opinions on the use ( and over use, and over elaboration of ) traps.

Personally, I'm a simulationist at heart - so in my campaign worlds, that sort of thing would never happen, as there's no plausible logical reason to create elaborate lethal mazes for the sole purpose of testing or amusement in a "real" world, and anyone who has enough resources to build that sort of thing, and is insane enough to actually do it, will likely have their resources seized by more focused and more stable bad guys. Sorry - but Hitler would stomp Dr. Evil.

Also, as a Player I prefer a plausible and logically consistent world  ( given some axiomatic changes to the basic nature of the world: magic exists, Gods exist and intervene semi-directly in the world,  non-human Intelligences exist and somehow no one species has genocidally wiped out everyone else yet, etc. ).

That's why I find things like Grimtooth's Traps to be annoying.

DO NOT DIS THE GULMTEETH!