What have been the most fun/impactful magic items you've found/made for your players?

Was it a weapon?
Was it a strange tool?
Was it something on the border of powerful and useless?
Was it something they absolutely wouldn't have expected?

What was it?

I didn't make them but personally I love these:
Deck of Many Things
Planar Cube
Immovable Rod (Matt Mercer HB)

The best one I can think of recently is a bag of holding that is sentient and wants to and is capable of eating anything we put in there. So, using the bag is often a negotiation and we tend to keep the bag happy by feeding it the stuff we kill. It's really developed a taste for giant. And it occasionally pipes up asking to be fed, freaking out NPC's that happen to meat us. err... meet, I meant meet.

Hmm. It's a tough call, as I enjoy creating items specifically for a given player or campaign.

I think one of the most impactful was a wand that randomly granted a benefit--maybe the spell was cast at a higher level, or there was a penalty to the save, or whatever--but one of the random options was that it could wind up functioning as a wand of wonder. And I didn't limit myself to the table in the DMG. There's an old Dragon article that had four different tables, and it could potentially (and randomly) roll a result from any of them.

The wizard who had the wand once died because of it (though he was later raised). See, one of the possible random results, from one of the charts, was that the wielder and the target switched places.

He happened to roll that result while trying to cast a spell on a beholder that was floating 120 feet in the air.

Sam Gamgee's Cooking Utensils.
You get a bonus to any CHR checks that involve cooking a meal as part of influencing somebody. (The 5e version is a set of Cook's Tools and grants Advantage on the skill checks.)
The frying pan can also be used as a Club +1 in a pinch.

Anything that can be used to manipulate the environment is 'better' (for this purpose) than a weapon, no matter the weapon's properties / powers.
Look at the list of stuff that is in a Burglar's Pack, and ask yourself, "How can I get into trouble using something in here?"

Things I've gotten as a player:

Rod of Security. Me and my party just got out of a dungeon, having barely escaped, only to get caught by a side bad guy that we had escaped twice inside the dungeon (I had a tendency to wiggle out of my DM's traps). The guy grabs my character, shoves a sock in his mouth (to keep him from talking more of his men into attacking him), and tells me either I give him the magical items we found in the dungeon or he kills us. I ask the DM "Is my hand free?" He says, "Yes, please try something." I said, "Ok, I reach around to my pack, grab the Rod of Security strapped to the outside (I had specified this beforehand) and press the button. Me and my team are instantly transported to a Beautiful Desert Oasis, complete with gorgeous serving girls willing to wait on us hand and foot." Dm says, "D*** it, Tony, you get out of everything!"

Frostbrand Rapier: Became my character's iconic weapon. Considering he was rogue who could halve damage when he FAILED a dex save, halving fire damage even further was amaz-ah-zing.

Best items given out as a DM:

Vorpal Sword to a lvl 8 Champion Fighter. They all decide to take a long rest in the middle of a Dracolich's lair, because they have Leomund's Tiny Hut. They didn't leave a watch. They wake up surrounded by skeletons and a Dracolich, who's head is inches from the top of the 'hut', waiting for it to go down. They all get ready, drop the hut, and the fighter wins initiative. Crit. Dracolich's head goes rolling.

Then they the Dracolich gets a new body and attacks again. They get away, barely, only one surviving with enough hit points to get them all to the Astral Plane using his Staff of the Magi I gave him (I wanted everyone to have legendary items). They find the Dracolich's secret chamber with the Phylactery. The Dracolich is obviously waiting for them, and prepared this time. He activates his Fear, and the Fighter fails his save. Yes! He closes in, the fighter attacks with disadvantage. First attack of the battle: 19. And 20. He's a Champion. He crits on a 19. He lops off the Dracolich's head right as the party Warlock, with the Staff of the Magi, casts Banish on the Phylactery. The Phylactery is not on the same plane of existence when the Dracolich dies. Therefore, it is snuffed out. Truly dead. OMG. Huge battle planned. Done in moments.

Vorpal Swords are awesome, but even more so in the hands of a Champion Fighter. I know technically the sword doesn't cut off a head on a Crit, it has to be a roll of 20, but I find that a poor distinction and have no qualms with changing it to work on a Crit instead of a roll of 20. I suppose this would be even worse for an assassin...

The Jade Pike (Bronze pole with a jade-stone spear head)

Requires attunement by a barbarian, valor bard, cleric, or fighter.

+1 to attack, +1d6 lightning damage.
The pike has 5 charges (recharge on long rest). Once per turn, when you hit a creature with this weapon, you can expend one charge, and the target takes an additional 2d6 lightning damage.

Yeah, it's simple, but considering the fact that it was the very first magic item I ever handed out to a player in 5e, it has a special significance to me.

The Scalmagdrion

A spellbook that was actually a summoning codex for a Dragon which could teach the user spells - like a mentor... as long as you earned it's trust and respect.

I came up with the idea in a 1st Edition D&D game in the very early 80's in a game I ran with school friends.

I later bought an official game supplement (Magic of Faerun perhaps - I forget) and saw my creation in print.

As far as I can tell, one of my players from that game worked on the book and used the idea.

I think that was the greatest compliment I ever had from a player - even if it wasn't directly delivered.

Soemthing my players took to calling Mr. Hat. It can give dreams and weird insights of the worlds or of the draconic prophecy from perspective of the Far Realm. It has helped interpret clues, find treasures, or just freak the players out.

Magic Items currently created for this campaign (running form level 3 to currently level 12):

Alis' Thundering Bowstring: Add 2d6 thunder damage to an attack and push the target away 10 feet 3x a day. (Uncommon, no attunement)

Raughley's Roaring Rampart: +1 Plate Armor with a Lion's Head on the front. Once per long rest, may animate the Lion and have it give the equivalent of a bless spell (+1d4 to all attack rolls and saving throws) to all party members within 30 feet. Also has a save effect to grant disadvantage to attacks for all enemies within 30 feet. Lasts until the end of his next turn. (Very Rare, requires attunement)

A Brother's Love: Made by Raughley's brother just prior to his murder, this +1 shield can be used as a ranged weapon dealing 1d8 damage with a 30 foot range. (Rare, requires attunement)

The Blood of the Twin Sisters: Two stones, one polished white, one polished black that orbit the head of Alis', the actual blood of twin deities of good and evil. Needed for a ritual by the BBEG to achieve apotheosis, taken after a hard fought victory over a beholder. They have a bunch of powers, the two most powerful being +2 wisdom and the ability to cast raise the dead 1x per week without spell components. (Artifact, requires attunement)

Naegrathand's Wizards Robes: Grants Nag the benefits of a mage armor spell while being worn. (Rare, requires attunement)

Xal'Talos, Great Axe of Entropy: A sentient vestige of power that was wielded by the party's barbarian, Heimdall, Son of Anders. Literally part of the god of entropy, Feinrod, cut from his body after a visit to the far realms. At stage 1, the axe functioned at +1 that could grant haste 1x per long rest. At stage 2, the axe bumped up to +2, still granted haste, and granted flight once per long rest (not at the same time as haste). Xal'Talos left the party after Heimdall tried to destroy it when Xal'Talos started exerting more and more influence over him. (Vestige, requires attunement)

Heimdall's Spinner Belt: Basically a +1 Cloak of Resistance in belt form.

Vestiges are a category of magic items created by Matt Mercer on Critical Role, I would assume based on Weapons of Legacy from 3rd addition.

There's two magic items that have featured prominently in my current campaign:

The ring of the ram: The perfect item to hilariously punch the big bad into the lava, again and again... and again... and yet again! Classic!

The jar of magical fortune cookies: This custom magic item contains a limited supply of fortune cookies, that provide the players with a cryptic clue or advise for their current situation. They are always helpful, though not always correctly understood. The players started out with so many cookies, that they wasted quite a lot of them for less than urgent advice (as I knew they would). Now that only a handful are left, they are starting to realize that they may have underestimated their value, and should use them more sparingly.