Ukrainian seems right in between Russian and Slovak to me, usually more understandable than Russian, but sometimes less due to loanwords (like harno).
Vocabulary is hardest in Slovene for me due to false friends and in Bulgarian due to loanwords and mutated forms of basic words.
I suspect understandability of Czech for other Slavs than us is also affected by their tendency to speak ridiculously fast and some Czechs elongate even short vowels to ridiculous degree (mainly central Bohemia).
