How can they be more Slavic than Belarussians for example when they have R1b 28% ? This haplogroup have nothing really with the Slavs. No offence to the Czech members but that is actually what it is.
If i have to say, Belarussians and Poles are the “purest Slavs” according to genetics. Croats, Bosnians (all ethnicities) as well. But, Serbia have a lot of “E” Albanian component. “Serbian DNA Project” proved that.