Languages closest to Russian
Russian is the most-widely spoken Slavic language with over 250 million speakers. It belongs to the eastern group of Slavic languages along with Belorussian and Ukrainian. Both these languages could be considered closest to Russian (although, when I hear Belorussian, it sounds to me as being more similar to Russian than Ukrainian). All three eastern Slavic languages are written in Cyrillic script.
The other two groups of Slavic languages are western and southern. Bulgarian, Macedonian, Serbo-Croatian and Slovenian are the main languages of the southern group while Czech, Polish and Slovak are the main languages of the western group. The southern languages are written in Cyrillic (or both Cyrillic and Latin scripts such as Serbo-Croatian). They are more similar to Russian than the western Slavic languages, whose script is Latin.
In southern Russia, the Cossacks know Balachka, which they consider as a language but which is largely considered as a dialect. It is similar to Russian and Ukrainian. Rusyn, a dialect spoken in small numbers in Ukraine, Czech Republic and Slovakia, is also considered as belonging to the eastern Slavic group of languages and is similar to Russian.
In the Soviet prisons, the prisoners evolved a code-language of their own based on slang words in Russian. It was called Fenya. Many Fenya words started getting used in Russian language over time. For example, when one wants a cup of strong tea in Russia, one jokingly asks for ‘Chefir’, which is a word invented by Fenya speakers.