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.

First of all they are south slavic, not west. I'd choose bulgarian.
Pros:
lack of cases
english-like grammar
90% of vocabulary are similar to russian(bulgarian is even easier to understand for russian than ukrainian that is considered to be part of east slavic languages(which includes russian)).
cyrillics
Cons:
you won't be able to understand other slavic languages unless you learn their declensions and cases(considered to be the toughest part of slavic languages)