Use a 2 part Teak cleaner. Best for anti-slip would be to leave it bare, repeat as needed (probably yearly).
Worst for anti-slip would be Varnish/Cetol - will require stripping/sanding, re-doing very couple years, total PITA especially around the carriage bolt heads.
Using a pressure washer will work OK & will remove the softer wood, leaving high/low spots, which is really good for anti-slip.