Give me your greatest puzzles

Quote from noonoothegreat37816 >>

A library that has books and they need to be sorted into categories 

 I actually contemplated doing this one for a while (ever since someone on D&D Beyond was looking for puzzle ideas for Candlekeep.

I would give them a set of 8 fairy tales, each of which corresponds to one D&D school of magic, and they unlock a hidden room or treasure by placing each book on the correct shelves.

This is what I've got for fairly tales. I'm not sure of all of them, and am open to other suggestions.

Abjuration - King Arthur (because they seem like paladins, and paladin magic is heavy on adjuration. This is my weakest match.)

Conjuration - Aladdin

Divination - Snow White ("Mirror, mirror, on the wall...")

Enchantment - The Pied Piper of Hamlin (Sleeping Beuty should also work, unless the thorns and dragon cofuse stuff)

Evocation - Greek mythology, with a picture of Zeus on the cover (my other really weak one)

Illusion - Leprechan story (I don't have a specfic story here, just put a leprechan on the cover of one book)

Necormancery - Frankestein

Trasmutation - There are a hundered, so choose any of the following: Rumpelstilskin, Puss in Boots, Beauty and the Beast, Cinderella, The Frog Prince, The Swan Princess, and so many others.

I would also not tell them what they have to do, just leave the books on the table, and have an open spot on each bookshelf. The bookshevelves aren't labled as schools of magic, but have books on different spells on each. (You can find spells for each school easily.) In general, I belive that with D&D puzzles, figuring out what to do should be part of the puzzle. 

Necormancery lol

That’s awesome, I’m gonna try that with my players and report back see how long it takes them

All great. 

Would love to also here how DMs factored player Int, investigation and/or other skills/abilities, into the puzzle solving. If at all... 

A sign that reads "need blood" written with blood and a door with no handle, since that there already are blood on the sign they don't need to do anything drastic, they just need to open the door. Works every time 

I am creating a campaign where the near-end of the quest is mostly puzzles. My party is actually smart, so I need this. Thank you for the explanation.

This is how my homebrew campaign starts, but it doesn't have to be specifically that.

Each of your players wakes up (or finds themself) in a stone room containing a bed, all of their belongings, and a window that has bars on it. There is no door, but the window— were it not for the bar— would be large enough for them to walk out of. On the windowsill, there is a series of runes, each looking newer than the last. The oldest one is completely dull, and all of them have been painted or dyed except for the very newest one, which glows brightly. They may or may not recognize any of the older runes, but the last one is the first letter used to write their name in their native language. If they press their finger to, lick, or otherwise touch the glowing rune, the bars slide away, and a stone platform slides out on the outside of the window. If they remove their hand/tongue/whatever, the bars slide back into place, and the platform slides back into the tower.

The goal is for them to look through their things and leave something in the rune. It can be paint, ink, blood, I've had someone use gunpowder out of one of their bullets before. Ashes from a match. If they don't have anything in their equipment they can use, they can tear or cut open the pillow from the bed, and use stuffing.

Once they have something permanently inside the glowing rune, the platform and window stay open, so they can walk out and use the stairs on the outside of the tower.

my greatest puzzle so far was an ancient blue dragon's lair, that was riddled with lines of gold coins fused together to form electric circuits.
the entire dungeon was a series of circuit puzzles of increasing difficulty, where the players had to turn revolving platforms and close alternating circuits in order to open certain doors and activate certain elements in the dungeon. the last room had a circuit that couldn't be completed using the setup of the room alone, and they had to figure out what they had on them that they could use to conduct the electricity to the correct line (or even form a chain themselves, and take some lightning damage to open the final door)

Quote from Spikepit >>

Each line (after the first) states what you have read in the previous line.

1
One 1
Two 1(s)
One 2 and One 1
One 1, One 2 and Two 1(s)
Three 1(s), Two 2(s) and One 1

Therefore the answer is

One 3, One 1, Two 2(s) and Two 1(s)
13112221

1
1 1
2 1
1 2 1 1
1 1 1 2 2 1 <= doesnt this have one too many 1s in front?
3 1 2 2 1 1
x x x x x x x x

Quote from SunIsGettingRealLow96589 >> 

1 2 1 1
1 1 1 2 2 1 <= doesnt this have one too many 1s in front?

 No it has the correct number of 1 to describe the line right above;

One 1

One 2 

Two 1

Some of my favorites have to be the most obvious puzzles ever. I think it took my party three hours to figure out they needed to ask for passage in rhyme to get through a door. Three hours. It took them only two hours, though, to figure out a puzzle I had set up where they were in this dungeon and there was no exit, it seemed. The only other person there was in one of the cells and had gone 'insane' from a bunch of 'chickens'. There were no chickens; you just had to cluck to get through the door.

If you have more musical players, you can do something where there is a grid on the floor, and each tile plays a different note (cdefgab), and they have to spell out a word.

My absolute favorite just to mess with players is describing a MASSIVE door that has chains and padlocks all over it. It's not actually locked, but it does take some heaving to get it open.

Quote from SilentArrow2312 >>

If you have more musical players, you can do something where there is a grid on the floor, and each tile plays a different note (cdefgab), and they have to spell out a word.

I did something like this one, with a clue written on the door: "I open only for the silent dead," and the players needed to cast Silence on the room before playing the notes.  (They found a scroll of Silence, and an enchanted ocarina, elsewhere in the dungeon.)

There's a riddle written on the inner door of a seemingly abandoned barbican:

"No answers if you question me; I'll question you instead. But if you shout I'll answer thee, To say what thou hadst said."

Making too much noice will alert the goblins and they'll attack through the murder holes that surround the players.