Western Slavic Languages

Most of us know that this includes Serbian, Croatian, Slovenian, and Bosnian. I know that S/C/B are very closely related, but the question is how related are they? Are they close enough that if I learned one would I understand the other ones rather well, or are they just mutually intelligible, and learning one would get me by in the other countries? If so, which one would be the best to learn. In an example, they say that Norwegian is the middle language, it shares vocabulary with Danish, but phonology with Swedish. How is it with West Slavic languages? I'd like to know, because learning one seems pretty attractive if it's 3 for 1.

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'Russian tongue (language)') is the most geographically widespread language of Eurasia, the most widely spoken of the Slavic languages, and the largest native language in Europe.

Over a quarter of the world's scientific literature is published in Russian.[4] Russian is also a necessary accessory of world communications systems (broadcasts, air- and space communication, etc).

Hence, the language is one of the six official languages of the United Nations.