In darts, what is the highest possible checkout with a single dart?

For a single dart the highest checkout is 50 points as a bullseye.

The checkout is the name given to the last three darts a player throws during a game of darts. Every game starts with the score set at 501, and every score a player achieves with three darts is deducted from the total, until it reaches zero. The difference as far as the checkout is concerned is that the final dart has to land on a double. If the remaining amount is 36, for example, the player has to checkout by hitting a double 18.

https://www.betting.net/help/atoz/h/highest-checkout/

It depends on the type of darts game being played. Like cards, there are many variations on how darts can be played. Just a few:

"Shanghai" in which the highest single dart 'checkout' could be 60 (triple 20) which would score 120 points for the three darts thrown..
https://www.dartsconnect.co.uk/why-is-shanghai-called-darts/

"Cricket" depends on whether or not you include the 'Bullseye' as part of the game - highest 'checkout' = either 20 or 50.
https://www.wikihow.com/Play-Cricket-Darts

Where I come from (Western Australia) there is a darts game called "Circles" where the object is simply to score more than the previous player. Although highly unlikely, you might win the game with a score of 1 (if the previous player missed the board with all three darts). The highest 'checkout' with the last dart would again be 60 (after having thrown two other triple 20s!!) to beat 177 - the next highest possible score with three darts.

Here is a link to a website that lists 36 different games of "darts".
https://dartspin.com/dart-games/