My first language is Russian despite I am ethnic Belarusian. It’s not uncommon for many Belarusians.
I've heard Slavs of different ethnicities speaking in Russian : Bulgarians, Serbs, Czechs (Czech president speaks Russian , for example), Slovaks, Poles, Macedonians, western Ukrainians who are native Ukrainian speakers. But never Croats, Slovenes or Lusatian Sorbs. Although, I've heard Sorbian children speaking Sorbian with German accent once.
Ukrainians speak speak perfect Russian with obvious Ukrainian pronunciations. Among southern Slavs Bulgarians speak and pronounce Russian well. Especially older generation. But my favourite accent is Polish when Poles Russian. I am not sure why I find Polish accent pleasant to my ears.
Which Slavic accent do you like when those Slavs speak in your native Slavic languages? Or the sound of Slavic languages, if you haven’t heard other Slavs speaking in your language?