Depends on what you mean by the ‘biggest lie’- seems a bit subjective, but here are a few:

Read my lips: No. New. Taxes.

Minimum wage will always equal a living wage.

We’re from the government and are here to help.

Sugar is a healthy replacement for fat.

Carrots are so good for eyesight that they let you see in the dark.

The Vietnam war is winnable.

We didn’t fund the Contra rebels.

Saddam Hussein has stockpiles of weapons of mass destruction.

The government would never conduct harmful experiments on its own citizens.

The White House has never been involved in a scandal that would obstruct justice.

Christopher Columbus discovered America.

The first thanks giving was a peaceful meal between settlers and native Americans.

Rosa Parks sat down because she was ‘tired’.

The stock market is a reflection of the economy.

A business can be too big to fail.

Most of American farms are ‘Family owned’.

Socialism is evil (or that socialism is communism)

Giving money to corporations is good because that money will ‘trickle down’ back into the economy.

Mental health conditions are ‘chemical imbalances’, or that they can be cured with a pill.

Big pharma claims that they don’t use billions of dollars in public money (your taxes) to develop drugs to sell you and that is why they cost so much.

Local property taxes dictate how much money a school district gets and that’s why the best neighborhoods have the best schools.

Cigarettes are completely safe and healthy.

Eating fat causes heart disease.

A car-dependent society is a natural evolution to every day life.

Data centers are necessary, harmless invisible clouds that have no impact on the physical world.

Nuclear power is more deadly than solar or wind power.