A shoo-in is a guaranteed winner.
This noun phrase first appeared in the 1930s in the context of horse racing. When there was a predetermined winner in a horse race, jockeys would hold their horses and shoo the winner in. Shoo means to drive something away while you yell, "Shoo!" — like the way you might shoo flies. Shoo-in is often used in politics — as in, "she's a shoo-in for mayor" — but it can refer to anyone who's clearly going to be number one or come out on top. Although the expression comes from corrupt horse races, today it doesn't carry any of the stigma of cheating.
Either of these sentences could be shoo-in for best usage example:
"Parasite" is also a shoo-in for best foreign film. (Seattle Times)
They are nine games ahead of the second-place Raptors and are a near shoo-in for the best record in the NBA, barring crushing injuries or an inexplicable collapse. (The Guardian)
Eggcorn alert! Shoo sounds just like shoe, which is why the phrase is sometimes misspelled as shoe-in. This mistake is common; perhaps people think they put a shoe in somewhere to win. You can find this misspelling of shoo-in even in respected publications:
Utah Rep. Jason Chaffetz announced Sunday he is running to replace John Boehner as speaker, challenging House Majority Leader Kevin McCarthy who seemed a shoe in to ascend to the House's highest post until the weekend. (Time)
Pittsburgh is a shoe in as long as Alleghany county, which controls the modern underused airport, blows it with high taxes. (New York Times)
Don't put your foot in your mouth — remember that a shoo-in is sure to win.