What bean species are best for baked beans?
If you live in Poland, you should easily find the "normal" white ones in your supermarket. They should be good for baked beans, although the British version uses white beans that are a bit smaller. 

For more American style beans, you could also use the pinkish-brown or spotted type.

There are so many types of beans, that I can't give a specific "name". Stick with the smaller ones, as they have less starchy parts and usually keep their shape better.