the best, the finest in Polish

How would i say k1s finest!

dzięki

k1s? What's that?

Its a kickboxing fighting sport based on thai boxing that is in a ring and with boxing gloves. There is a bardzo dobrze polish fighter named Poul Slowinski who fights 4 them in this orginazation.

najlepsi w k1s (if k1s is a sport, or another activity)
najlepsi z ks1 (if k1s is an organization, club, place, locality)

In both cases I used plural form

best fighter in k1s - najlepszy zawodnik w k1

I think that in this case it is a singular and najlepszy should have been used.