Lethal Vapors + Grand Abolisher + Teferi's Protection
I've never tried to pull this off in a game, but its the funniest combo I know. I call it "win the game by removing yourself from it" or "how to play magic without playing magic"

You start with [[grand abolisher]] out. This is important to keep others from doing the trick you're about to do. Next you play [[lethal vapors]]. The important part of this card is the activated ability. You put 1 million copies of that ability on the stack, skipping your next 1 million turns. While those are on the stack, you cast and resolve [[teferi's protection]]. So now, until your next turn, your life total can't change, and you have pro-everything, and your next turn is 1 million turns from now. From here, the game plays out with other players as normal, until its down to just one. They continue taking turns until they deck themselves, then you win by being the last player "technically" in the game.

You can still lose to certain things like global mill effects, alt win con cards, or commander damage combined with "damage cannot be prevented" effects. Your opponent(s) can also get around your million turn decking strategy by having some way to infinitely shuffle one or more cards back into their library. If they do that, 1 million turns later, they will have a boardstate consisting of (theoretically) basically every permanent in their deck, and a perfect hand, and you'll have whatever you had when you did the combo, so you'll probably lose.

However, in "normal" circumstances, where your opponents are trying to win through some kind of damage and they don't have a way to keep from decking themselves, you'll win.

Bonus points if you pull this off in a game store, go join a different pod and start a game while you're technically still in the first one, and win the first one all while playing a new game of magic.
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Kaervek's Spite + Academy Rector + Barren Glory
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Caged Sun + March of Madness + Maskwood Nexus + Life and Limb
Animate a [[Caged Sun]] with [[March of the Machines]]. Make it a saproling with [[Maskwood Nexus]]. Make it a land with [[Life and Limb]].

Your Caged Sun is now a 1/1 Saproling Forest.

Now tap it for mana. This is where the fun begins.

Because Caged Sun triggers when a land you control adds mana, normally it doesn’t trigger itself, because it’s not a land while keeping all its abilities. However it is now, and it will add mana, then see that a land added mana, then Caged Sun makes mana.

Normally when Caged Sun adds mana it ends there because it wasn’t a land. But now, it’s caught in a loop adding mana to your mana pool. With infinite loops, you can respond and stop it potentially, else the game is a draw.

Here though, we have an infinite loop that adds mana, something that doesn’t use the stack. No one can respond to it because the rules say no one has priority until Caged Sun is done, but it never will be.

All players are stuck. No land plays, split second, nothing. You can’t even pass priority. Congratulations, you broke the game in a totally irreversible way. All while Caged Sun sterling engine is busy making infinite mana.
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Syr Konrad, the Grim
Painter's Servent
Grindstone
Kozilek, Butcher of Truth

[[Syr Konrad, the Grim]], [[Painter‘s Servant]], [[Grindstone]] and ~20 Creatures in your deck including [[Kozilek, Butcher of Truth]]. Mill your own deck to kill the table 😏 obviously this is jank af but i made someone‘s day playing this in a mtgo commander game and this post brought this moment back to memory!
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Chicken à la King
Feywild Trickster
Artificial Evolution || Arcane Adaptation || Maskwood Nexus

Infinite dice to roll, infinite 1/1 flying tokens. Only problem is keeping track of the +1 counters on the tokens.
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Mycosynth Lattice
Vandalblast

Daretti, Scrap Savant deck
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Illusions of Grandeur
Donate

[[Illusions of Grandeur]] + [[Donate]] to make opponent lose 20 life :P the cards are expensive though.
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Norin the Wary
World Fire
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Hive Mind
Eternal Dominion

[[Norin the Wary]] + [[Worldfire]]. Norin will be alone on the battlefield for a while, but he can't actually kill anyone, even though everyone's at 1 life.

I'm also a fan of [[Hive Mind]] + [[Eternal Dominion]]. Everyone just plays each others' decks.

I have both in my EDH decks, but for some reason I've never pulled off either. Probably because of the mana costs...
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Ardenn, Intrepid Archaeologist
Captain's Hook

I have an [[Ardenn, Intrepid Archaeologist]] deck that includes a [[Captain's Hook]] solely for the purpose of using Ardenn to attach it to an opponent's creature, and then immediately equipping it to one of my creatures after combat, thus destroying my opponent's creature. Let them feel the joy of being a pirate for one combat before ending their life. Incredibly inefficient, but good fun.
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Phage the Untouchable
Fractured Identity
Hive Mind
Intervention Pact
Pact of Negation
Barren Glory
Oblivion Ring
Worldpurge
Near-Death Experience
Crovax, Ascendant Hero

I think you'd like [[Phage the Untouchable]] + [[Fractured Identity]].

I have a commander deck whose theme is alternate win conditions (which sadly doesn't contain the above combo as its identity is WU). Some highlights are:

[[Hive Mind]] + [[Intervention Pact]] or [[Pact of Negation]] - each opponent casts a copy forcing them to either pay 1WW or 3UU on their next upkeep or lose the game, whether they're in the right colours or not.

[[Barren Glory]] + [[Oblivion Ring]] + [[Worldpurge]] - when Worldpurge bounces all your permanents only the Barren Glory will be left. Worldpurge allows you to choose "up to" seven cards to keep in your hand; choose 0 so that you have no cards in hand.

[[Near-Death Experience]] + [[Crovax, Ascendant Hero]] - if your life total is odd, activate Crovax as many times as you like to make it 1.

Here is the list: https://manastack.com/deck/you-win-the-gam
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urza,prince of kroog
Honestly, I really want to build this as Ornithopter tribal. 0 and 1 mana artifact creatures that suddenly become aggresively costed attackers, many with evasion. Then you use anthem artifacts to boost them even further, and copy those.
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Jeleva Big Spells - Combo off with enter the infinite and nexus of fate for infinite turns.

Hurkyl Taking turns - Take 20 turns with stone calendar or court of cunning.

Wilfred Stax - phyrexian unlife combo w book of exalted deeds combo

Sivitri clones - tutor kokusho, play damage doubler cast kokusho spam phantasmal image

Pantlaza Prison - 90 lands uses scapeshift and equivalent with, glacial chasm, opal palace and beacon of immortality to stall.

Breya superfriends - Good superfriends colors. Blockers and removal at the ready
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--badcombos--
Hive Mind
Endless Swarm
Coat of Arms

My wincon for the Storm, force of nature deck I built. I’ve managed to pull it off once and it was so much fun beating each other up until the end of the game