Polish, Czech, Slav... similarities?

Good evening.

Purely as a hobby, i've decided that i'd like to pick up a European language (I already speak a couple Asian languages).
I initially wanted to learn Czech, but i was comparing populations that speak languages in Europe and found that Polish outnumbers it and that it also has a lot in common with slavic languages.

tl;dr:  Anyone who speaks Polish or Czech, or any combination of those Slav languages, are there similarities?  'ppreciate your comments.

I'm from Croatia.
Czech and Slovak are very similar to each other (I believe it's called "mutual intelligibility"), just like Croatian and Serbian. I have a hard time understanding Polish though. It's somewhat different than the languages I mentioned above. Basically, I(atleast partially) understand every Southern and Western Slavic language (except Polish).
Eastern Slavic languages (Russian and others) are something completely different + you'll have to learn the Cyrillic alphabet (which is also used in Bulgaria and Serbia btw).
Anyways, I suggest Czech.