Joms-Vikings, the best fighting force on the Baltic (as acknowledged by Vikings themselves), were mostly Slavs.

In 965 AD the Viking Harald Gormsson (a.k.a. Bluetooth) married a
Wendish princess and built a stronghold at Jomsborg on the Baltic
Coast near the Wendish trading center of Wollin. Here a famous
brotherhood of Vikings (i.e., the Joms-Vikings) founded by Harald's
son, Svein Forkbeard, is supposed to have collected, fighting with
the Wends against foreign invaders on land and sea. The Joms-Vikings
were essentially a warrior cult (a sort of Pagan Knights Templar) who
lived in barracks under harsh discipline and trained constantly in
hopes that they would die in battle so as to join Woden in Valhalla.

According to Adam of Bremen Joms-Vikings were Slavs, as confirmed by archeology.