Favorite trap

[Laserlight]

20ft deep pit with a spring-loaded trap door, so it drops when something weighing more than 50lb crosses it and closes itself when the victim falls in. There aren't any light sources in the pit, but there are two rust monsters....

(Our paladin fell in, couldn't see, still hit a rust monster and nearly killed it, but then heard it whimpering and went over to pet it)

[Lunali]

From a published oneshot adventure full of very lethal magical item traps that I wish I could find again:

Crawling tunnel that sticks out partway up in the wall of the next room, last 10 ft of the tunnel is inside an enlarged bag of holding stretched over the exit. If the first party member or two has a bag of holding/portable hole/etc or if they decide to cut their way through the cloth blocking the exit...

Others from that adventure that I remember:

Entrance to the dungeon at the bottom of a well (with water in it) directly over a patch of dirt with a bag of beans planted underneath awaiting watering.
Decanter of endless water + party member(s) inside a force sphere, decanter in a depression in the floor where if disturbed would become a rocket in addition to the drowning threat. Unfortunately forgot the details of the trap trigger, but before 5e the decanter stayed on until turned off.

The whole point of the dungeon was having a lot of magical items as traps with a high likelyhood of the item being destroyed in disarming the trap.

[Requilac]

This is one I created as a DM

The PCs find a magical portal to a demiplane. You can see which room you are entering through the portal, like a window. The portal appears to lead to a simple brick chamber, roughly a 20 ft. cube, with a gateway on the opposite end of the portal. When the party steps through though, they realize that the floor was actually slanted sharply downwards for 60 ft. and only looked flat from the portal due to a visual, non-magical "illusion". The floor slopes down into a bunch of gems which are spewing flames. The gateway that you saw through the portal is still there, so you have one round to reach that gateway and enter into a new room before you start taking fire damage.

It was quite a fun trap, but a little complex.

[sithlordnergal]

So far my absolute favorite trap has been from Tomb of Ahnniliation. You find this throne that looks normal, but if you touch it you have to make a Charisma saving throw. If you fail, like our Moon Druid did, you gain 50 temp hp and have to attack the nearest creature with everything at your disposal.

The Moon Druid then set off a second trap by attacking some peaceful undead that were close to him, waking up an undead T-Rex. The party had to fight the T-Rex and a Moon Druid with two CR 3 Wild Shapes available to them. It ended up being the second deadliest fight the party had faced in ToA, only the Acererak fight was deadlier.

[Xetheral]

I'm a fan of vacuum-filled, sealed chambers coated with a flammable particulate (cooking flour works well). When the door seal is breached, inrushing air mixes with the particulate and triggers a clockwork timing mechanism connected to an incindiary. The PCs have a round or two to find the mechanism and stop it before the whole room turns into a fuel-air explosive.

The fire damage is bad enough, but the depletion of all the oxygen in a portion of a tomb combined with the overpressure and combustion products from the blast produces a serious suffocation hazard.