I decided that it would be fun to add a page of one hit KO's. There are a lot of ways to take someone down from 8000 to 0 in one turn. We don't count thing like - have two Summoned Skulls and a two Harpie's Brothers attack - because that is too unoriginal. All thee ones we list could be done on your first or second turn, if you get the right cards and your opponent has no monsters or traps to stop you. I'll start with the most talked about combo in the game.

Cards Required: 2
Turns Required for Setup: 0
Total Damage: 9000
Cyber Stein (Devil Franken in Japan) - Special Summon Blue-Eyes Ultimate Dragon, play Megamorph.
How it works: Cyber Stein's lets you pay 5000 Life Points to Special Summon a Fusion monster from your Fusion Deck. Blue-Eye's Ultimate Dragon's ATK is 4500. Once you've paid the 5000 Life Points to bring out the Dragon, you're pretty much guarenteed to have fewer Life Points left, so Megamorph doubles the ATK of Blue-Eyes, making it a 9000 ATK monster. All you have to to win is get 1 direct attack.

*The biggest downside to this combo is the HUGE cost. If you don't manage to pull it off early in the game, you won't get to use it. Other than that though, it's very powerful, especially considering that, in Japan, NONE of the cards are restricted.
Rating: 4/5

Cards Required: 3
Turns Required for Setup: 1
Total Damage: 10000
Scapegoat (during your opponent's turn) + Hayabusa Knight + United We Stand
How it works: With 4 scapegoats on the field and a Hayabusa Knight, United We Stand give Hayabusa a 4000 point boost, making it a 5000 point monster. You can then attack with the Hayabusa twice during the Battle Phase.

*The combo uses just one restricted card, so it's not too difficult to pull off. United We Stand and Scpegoat are both good cards, so you don't hurt yourself by adding them to your Deck. Hayabusa is good in the right deck.
Rating: 3/5

Cards Required: A deck built around it
Turns Required for Setup: Can happen before your opponent ever gets a turn
Total Damage: 8000+
Catapult Turtle + Magic Scientist
How it works: Magic Scientist allows you to pay 1000 Life Points to Special Summon a level 6 or less Fusion Monster from your Fusion Deck. The monster summoned cannot attack your opponent's Life Points directly. You pay 1000 to put a 2100/2200 ATK (using English Fusion monsters only) or 2300/2200 ATK (Japanese Fusions Monsters not out here yet) monster on the field, then launch it with the Catapult Turtle for 1050-1150 Direct Damage. Do that 7 times for 7350-7850, then launch the Scientist and Turtle for 8000-8500 total damage.

*This is the best 1 hit kill combo I've ever tested out. All you need to win the game before your opponent gets a first turn is a Cannon Soldier and 2 Last Will, a Painful Choice with Monster Reborn and Premature Burial, a Graceful Charity with a Catapult Turtle and Last Will, or some other way to get the 2 monsters out 1st turn (there are several others). As you can see, this combo is incredibly easy to pull off, and, unlike Devil Franken, you don't ever have to attack. However, like Devil Franken, if you don't get the needed cards early enough in the game, you're out of luck as far as this combo goes, and chances are that you're going to lose the game.
Rating: 4.9/5 before restriction, 4/5 after

- I haven't done anything with the deck since Scientist has been restricted to 1. I only played the unrestricted deck in 1 tournament, and took 2nd in about a 45 person tournament.

Cards Required: 3
Turns Required for Setup: 0
Total Damage: Infinite
Gearfried the Iron Knight + Magical Marionette + Butterfly Dagger
How it works: Equip the Butterfly Dagger to Gearfried again and again. Everytime you play it, you add a counter to and power up the Marionette. Remove the counters 2 at a time to get rid of the monsters on your opponent's field. In the end, you have a monster with as much Attack power as you want it to have, and an empty field to attack.

*The dagger is restricted to 1, the other 2 are not. Because the Marionette is a Tribute monster with 2000 ATK (can't get it with a Mystic Tomato or Last Will), and Gearfried only has 1800 ATK, this combo isn't easy to pull off. However, when you do manage to pull it off, it's awesome.
Rating: 2/5

- For Exodia decks, you can replace the Magical Marionette with the Royal Librarian (level 4/0 ATK/2000 DEF), which allows you to draw 1 card for every 3 Magic counters you remove from it.

Cards Required: 4
Turns Required for Setup: 0
Total Damage: 8100
Red Eyes Black Dragon, De-Fusion, Polymerization, Summoned Skull (and Black Skull Dragon in the Fusion Deck)
How it works: Use Polymerization with Red Eyes Black Dragon and Summoned Skull to bring out Black Skull Dragon. Attack directly with the Black Skull Dragon first, then, before the Battle Phase ends, use De-Fusion from your hand to return it to the Fusion Deck and bring out the Red Eyes and Summoned Skull. Attack with both of those.

*This is one of the 1-hit ko's that I've tested fairly thouroughly. It's not all that great. It's really fun to pull off, but it's really difficult to get all of the needed cards in your hand at the same time. I played this deck in a tournament as well and I don't think I even placed top 8 out of 40.
Rating: 2/5

Cards Required: 1 (more if your opponent doesn't have the right card out)
Turns Required for Setup: 0
Total Damage: 8400
Magical Scientist (3 Mirage Knight in the Deck and 3 Dark Flare Knight in the Fusion Deck)
How it works: When your opponent has a monster with more than 2200 ATK, you summon the scientist and pay 3000 Life to bring out the 3 Dark Flare Knight. You attack your opponent's attack position GAF or Jinzo so that the knights are all destroyed in battle. For every destroyed knight, you can bring out a Mirage Knight. Have the first Mirage Knight attack the Jinzo/GAF/other. Since it's ATK is increase by the original ATK of the defending monster, you do 2800 points of damage. The other 2 Mirage Knight attack for the same amount.
- If your opponent doesn't have the right monster out, you have other options as well. You can throw in Crature Swap to give them the right monster. Also, you can choose to give them a Dark Flare Knight and have your Dark Flare attack theirs, giving you 2 Mirage Knight. Once Crimson Bird comes out in English, this combo will be better. It's the 2300 ATK Fusion. Because you can get it with Scientist, it's perfect bait for Creature Swap.

*Yet another 1 hit Ko I've tried out. Unfortunately, it's not all that awesome yet. Once we get the bird and a couple of other Japanese cards, it'll be awesome.
Rating: 2.3/5

Cards Required: 5
Turns Required for Setup: 0
Total Damage: 8100
Alpha, Beta, Gamma Magnet Warriors, Monster Reborn (or Premature Burial), and Valkryion the Magna Warrior
How it works: Send the 3 Magnet Warriors to the Graveyard from your hand to Special Summon Valkryion. Use Valkryion's effect to send it to the Graveyard and bring the 3 Magnet Warriors out to the field. Play Monster Reborn/Premature Burial on Valkryion. Attack with all 4 monsters for 8100.

*I've never tested this combo, but I have a friend who has. It's really not that great. In order to pull it off, you have to have all five cards in you hand or on the field. Gamma and Alpha can be be Summoned via Giant Rat or brought to your hand by Sangan, but there is no good way to get Beta. In thought, it's a good combo, but in reality, it's just too hard to pull off consistantly.
Rating: 1.6/5

For even more fun, why not list a few of the ways you could kill yourself. Some of these could be done in one turn, but not all of them. You use these combos against little kids so they will keep playing and not get discouraged about losing all the time. Or to show them what they shouldn't do.

Play Megamorph on your opponents BEUD then play a Confiscation to make your life points lower.

Change of Heart you opponent's face-up Amoeba.

Put Immortal of Thunder in Face Down Defense right after your opponent gets Change of Heart with a Cyber Jar or something. Tribute Immortal of Thunder during your next trun.

Use Devil Franken.... twice.

Flip Immortal of Thunder when Bad reaction of Simouchi is out, then attack your opponent's higher ATK monster with it.