Slavic languages, which group to go after?

Ok /lang/, I'm learning German and want to learn a slavic language, question is, which one?

I know the basics about slavic languages, bulgarian and macedonian are intelligible, croatian/serbian/bosnian are nearly identical, with some basic differences, and czech and slovak are intelligible.

I know that the obvious choice is Russian due to the home country's affluence in the world, but everyone learns Russian.
Poland doesn't suck too badly, but could be better, so its language isn't a terrible choice.

Which one would I learn to get the most in-the-middle language, so that if I wanted to, learning the other ones would be easier than say learning Czech, then trying to learn Bulgarian?

I want to learn one because I enjoy the way they sound, I just don't know which one to learn.

OP, protip:

Listen bits of many Slavic languages - at least three langs:
one East Slavic [Ukranian, Russian...],
one West Slavic [Czech, Polish...],
one South Slavic [Bulgar, Croatian...]

You don't care a shit for affluence, so you want to learn for the fun, right? So, learn the one sounds cooler for you :D

And, you know, if you learn a lang from one branch [East/West/South], it'll become very easy to learn another one from the same branch - and to some extent, from other branches too.

>>16
10 is probably talking about Japanese and Mandarin.