Officially, the best Wordle starting word is SLATE.
I say 'officially', because SLATE is the choice of WordleBot, an AI tool designed by the New York Times to help you improve your Wordle game.
WordleBot rates SLATE as 99/100 for skill, the highest score it awards. However, it gives the same ranking to CRANE, CRATE, SLANT, TRACE and CARTE, so any of those six would be a great choice of Wordle starting word.
Alternatively, researchers at MIT have calculated that the best word to start Wordle is SALET. Read more about that below.