God

"He is a force bigger than mom and dad put together, and you owe Him big."

- Lisa

God is the Almighty Ruler of the Universe.

History
He once attempted to flood Springfield, but stopped when Reverend Lovejoy showed the Springfieldians that God was the path to good, and also once chose to end the world. This time, He was stopped by Homer Simpson, who requested Him to turn back time and leave the apocalypse off for a few years. God has two aides, Colonel Sanders and Buddha, although he is seen with Jesus in Heaven. In an interesting side note, SpongeBob once appeared in Heaven alongside Buddha and Jesus. In one episode when asked if they should help someone, SpongeBob replied with a wave of the hand and said "screw 'em". God's voice appeared when Homer and Barney decide to join forces and Homer says "Not even God can stop us now!" After this God replies, "Oh No?" and dries up all the snow. It is also implied once that God has a tendency to cause train wrecks.

When Steve Mobbs went to Heaven, he became God's boss and refused to give him Christmas, his son's birthday, off.

Non-canon
God is also portrayed by Ned Flanders' hand.

Homer met God to say a word about family, but even Heaven is top notch destination, although Homer can't enjoy being in Heaven knowing that his family is suffering. On God's desk there are three pictures, God with Marilyn Monroe and Lincoln. God said He understands about family suffering because His son lived on Earth once, but He doesn't know what happened and he hasn't been the same since.

Because Homer riots all the time he is in Heaven, God consults Homer about the problem. Homer asks God to undo the rapture and await another time. God then agrees, but Homer ask God to do something else when He's undone the Apocalypse, where he must be shown as the Last Supper.

St. Peter was sitting on a couch with God in Heaven discussing with him who they could bring up. He and God listened to three stories and eventually decided to just allow anyone who is good up to Heaven.

God sold Heaven to Google. He then started to regret it. He also thought that global warming may be real.

The Simpsons Game
God serves as the final boss to The Simpsons. He uses previous enemies encountered as icons in a "Guitar Hero"/"Dance Dance Revolution" match, while the chosen song ("Rock you Like a Hurricane", by the Scorpions) plays. As the whole family, you have to make them hit the notes in a correct time to break the game stations and defeat the minions he sends. They will change when the station is broken. Each station is a parody of a popular video game console (i.e. Praystation is a parody of PlayStation and the I Am is a parody of the Wii).

Trivia

 * In most of God's appearances in the various Simpsons media, his face is covered up (presumably deliberately). There were, however, two instances where his face was fully seen; in the opening of "Treehouse of Horror XVI", and in the frame Ned Flanders turns round in "Alone Again, Natura-Diddily".
 * God has 5 fingers whereas other characters have 4.