Among west Slavic languages Polish is most similar to east Slavic languages
Polish people may be the closest to east slavs genetically and Belarusian and Ukrainian have similarities with polish due to heavy polish influence in their languages but I would say Slovak is closer to Russian. Why? Experience and from hearing Russian speakers who have ever had contact with the two tell me so. I worked in a hotel this summer and had a lot of contact with Russian guests. Sometimes they couldn’t speak German or English. My coworker would try to explain stuff in Polish but they would get jack. When I spoke Slovak to them they were able to understand. This was the case at least three times working this summer. A good friend of mine who is Russian (half Tatar) also has been in Slovakia and Poland and told me she understood far more from Slovak than polish.