Can't pick what Slavic language to learn...

Выбирай украинский язык!
Choose Ukrainian!

the most useless Slavonic language with Belarus.

Indeed, both Ukrainian and Belarussian are mere overemphasized Eastern Slavic dialects. Russian is a real living language both in written and spoken form.

A Belarussian told me recently that she cannot speak Belarussian, only Russian. I would estimate her to be about 32 years old. On the other hand, I have certainly met Ukrainians who can speak Ukrainian (while being able to speak Russian as well).