Spartacus was Thracian by birth therefore genetically the present Bulgarians are the most close to him. As for being a Serb, I would like to find any link but the time span and location makes it impossible.

What is noticeable in ancient history most of the surnames and some of the names have "us" as suffix regardless if they were Romans, Persians, Macedonians or Hellenes.   

I think it was because of the Romanisation of the population.

As for Spartak I think he is closest to bulgarians.