Star Trek: Discovery episode 5, "Choose Your Pain" contains the following dialog after they come to understand how the tardigrade interacts with the space fungal network:
TILLY: You guys, this is so fucking cool. ...
[regarding the swearing] So sorry.
STAMETS: No, cadet. It is fucking cool.
Whether this is the strongest language used in any Star Trek television series so far, especially given the intent to express amazement, not offense, given the instance of the N word (see this answer) is a matter of opinion.
