I would say that slovakian is probably the easiest to understand for polish person.
That’s true. Slovakian language is very similar to polish.
When it comes to Czech tongue, for a Polish speaker, it has to be the most hilarious language in the world. It's probably due to it's silly accent and some of their names for animals, items, products, professions, etc...
For example, let's take the word "pigeon". In Polish it's "goł±b", while in Czech it's "dachowy obersraniec", what in polish means more or less something like "something that is taking a shit on the roof". :)
I never hear Ukrainian.
I’ve met few Ukrainians, very friendly people, but what was really striking for me, that they only spoke Russian. It's kind of sad.