to me it was definitly YUGOSLAVIA until the '90s
it had Slavic anthem, slavic flag and slavic name.
Now I think of Slovakia when I hear Slavic, but most Slavic impression gives Serbia, particulary it's northern province-Vojvodina.
Becouse Vojvodina has 4 official Slavic languages, Serbian, Croatian, Slovakian and Rusyn.
In Vojvodina live Serbians, Croats, Slovaks, Rusyns, Czechs, Ukrainians and Russians.
that's my opinion