Calculations assume 3 opponents without life loss or gain.
In approximate order of winningness:
Deceiver of Form + Pathbreaker Ibex
gives trample, +8/+8, +16/+16, and +32/+32 to your army with just 3 creatures, which should be lethal. For some lethal trivia points, you should also inform your opponents that the plural for Ibex can be either Ibexes or Ibices.
Gisela, Blade of Goldnight does lethal damage to 3 opponents if you have 3 or more (hitting each opponent with 1), and flies over most creatures.
Biovisionary wins without attacking if you have 4 creatures.
Medomai the Ageless gives an extra turn for each creature you hit with. Drawback of being unable to attack next turn is avoided because your creatures become something else next turn.
Kolaghan, the Storm's Fury allows you to attack for flying damage equal to the number of creatures you have cubed, and kills 3 players if you have 6 or more (only 1 needs to connect per opponent).
Borderland Behemoth potentially deals lethal trampling damage with 6 of them.
Omnath, Locus of Rage burns your opponents to death if you have 7 of them and no Mirror Gallery. Kokusho, the Evening Star does the same if you have 9 of them.
Tidespout Tyrant removes your opponents' permanents if you have a few of them and some spells to cast. Mana Crypt + Sol Ring + Tidespout Tyrant
generates infinite mana, and with just 2 Tyrants you can bounce every single permanent your opponents have in the process. Mana Crypt or Sol Ring can also pair in this combo with Everflowing Chalice, Expedition Map, Skullclamp, Sensei's Divining Top, Fellwar Stone, or Thran Dynamo. Mana Crypt will also work with Lightning Greaves, Swiftfoot Boots, Coalition Relic, Scroll Rack, or Conqueror's Flail. Bloom Tender also works with all the above if hasted and you have at least 2 colors of permanents.
Rites of Initiation + Windreader Sphinx
draws 25 cards with 5 unblocked sphinxes and deals lethal damage by discarding 18 cards to Rites of Initiation, if one of the sphinxes is your commander (if not, you need 6 unblocked sphinxes). Windreader Sphinx can draw your whole library if you attack with 9 or more of them, which wins the game on the spot if you Flash + Laboratory Maniac
with draw triggers remaining.
Elesh Norn, Grand Cenobite should wipe your opponents' creatures if you have 5 or more, 8 of them attack for lethal damage when there are no blockers, and 9 of them allows you to kill someone with commander damage.
Each Enduring Scalelord will get infinitely huge if you have 2 or more, and can fly, but need a way to add +1/+1 counters on them.
Aurelia, the Warleader gives additional attacks for each creature, potentially dealing lethal damage in the air with 7 of them, but they are small and may get blocked. The Deceiver will trigger again before additional combat steps so you can scry and turn the Aurelias into better creatures if the right creature comes up.
Blightsteel Colossus is lethal to one opponent on its own, but in practice there will be blockers so you might need 6 or more to trample through.
Woodvine Elemental draws you cards for each attacking creature and pumps your trampling army, so 6 or more should deal lethal. However, your opponents also get to draw, so instant speed removal can be a problem if you have no Conqueror's Flail, Heroic Intervention, or Pact of Negation.
Ulamog, the Ceaseless Hunger should exile an opponent's entire library if you attack with 4 of them, even if they are blocked.
Gilt-Leaf Archdruid ramps you massively and takes an opponent out if you have 7 creatures.
Etali, Primal Storm casts up to 3 spells for each attacking creature so you should be in a very dominant position.
Krenko, Mob Boss doubles your army for each creature you have. If you started with just 4 creatures you end up with 64. If you have Hammer of Purphoros all those goblins can attack right away.
Utvara Hellkite squares the number of creatures you have by spawning dragons. Sharding Sphinx squares the number of creatures you have by sharding thopters.
Lord of the Void summons a creature from an opponent's library for each creature you hit with.
Mindleech Mass lets you cast spells from your opponents' hands, while getting rid of their nonland cards.
Beguiler of Wills allows you to steal all the best creatures for each non-summoning sick creature, with each theft allowing you to steal bigger creatures.
Keiga, the Tide Star sacrifices your creatures to steal the same number of creatures from your opponents if you don't have Mirror Gallery out.
The Ur-Dragon draws lots of cards and lets you put permanents into play.
It That Betrays steals multiple permanents when you attack.
Kozilek, Butcher of Truth and Ulamog, the Infinite Gyre annihilate your opponents' boards.
Angrath's Marauders has the same damage potential as Gisela with 4 or more of them, but does not have evasion or trample.
Grimgrin, Corpse-Born destroys lots of creatures without the drawback of not untapping. Thraximundar makes your opponents sacrifice creatures and grows bigger for each one you have.
Shauku, Endbringer exiles creatures that you want to get rid of, but cannot attack. Avatar of Woe destroys instead of exiles.
Emeria Shepherd returns permanents from your graveyard to the battlefield if you search for Plains with fetches or ramp spells.
Yore-Tiller Nephilim returns creatures from your graveyard attacking.
Dune-Brood Nephilim will generate a large number of tokens if they hit a player.
Vorinclex, Voice of Hunger ramps you insanely if you have a few of them, and also swing for 7 trampling damage each. Casting Sliver Queen after and pumping out lots of slivers is one way to use that mana, or casting Biovisionary + Rite of Replication
.
