This is the one I overlooked when initially making this list. Then, after typing up my initial #10 post, I was browsing around online and noticed a Vader song. After slapping my forehead in disgust, I promptly modified my list to make room.
1. Vader (death metal)
2. Behemoth (blackened death)
3. Vesania (symphonic blackened death)
4. Asgaard (gothic doom)
5. Lux Occulta (black/avante-garde)
I should note here that there is a lot of space between 1 and 2, and even more space between 2 and 3. Of course Vader and Behemoth pretty well dominate Polish metal, so I doubt that assertion comes as much of a surprise. In any case, you'll notice that I have officially reached the countries that have enough bands I like for me to do ordered top 5s now, and I'll carry on with those as I move forward with the list.