Biovisionary+Kiki Jiki, Mirror Breaker+Intruder Alarm

Auto win on the fifth turn

Conspiracy+Rooftop Storm

All your creatures cost 0.

Living Plane+Electrickery

Destroy your opponents lands for six mana

Grimgrin, Corpse-Born+Elemental Mastery

Infinite power and creatures.

Necrotic Ooze+Palladium Myr and Pili-Pala in the graveyard

Infinite mana of any color

Mycosynth Lattice+Darksteel Forge+Nevinyrral's Disk

Quite a bit of mana but let's you repeatedly wipe the opponents board (for one mana) will leaving your stuff alone.

Quillspike+Devoted Druid+Rites of Consumption

Repeatedly tap devoted Druid, place a -1/-1 counter on it, then eat that counter with quill spike and the mana you created. When Quillspike is big enough, play Rite of Consumption to kill an opponent.

Phyrexian Processor+Trostani,Selesnya's voice

Being able to dump 15 or more life, and then gaining it back immediately and creating the biggest creature on the board is awesome.

Laboratory Maniac+Temple Bell+Mind Over Matter

Quite expensive, but this works in multiplayer too, and an auto-win is still an auto-win

Hellfire Charger+Bear Umbra

Infinite attack steps

Duskmantle Guildmage+Mindcrank+Any damage spell

Basically a cheaper Sanguine Bond+Exquisite blood

Bloodflow Connesuir+Reveillark+Karmic Guide

Play and sac Reveillark, play karmic guide to return Reveillark and sac Karmic Guide then Reveillark, repeat.