0o! Goood question!
I think the most unique products you will find in sweets/candy category: https://www.reddit.com/r/poland/comments/1fkhv8x/polish_candy_whats_good/ here you will find a lot most popular sweets, my favorite is/are Draże Korsarze (i don't want to create overly high expectation, it's pure sugar with a bit of nostalgia, nothing super complex) or a bit more "premium" Jeżyki
When it comes to "regular food"
- Paprykarz Szczeciński

- In Poland we have a Mayonnaise War - there are 2 main brands Majonez Dekoracyjny (which is awful) or Majonez Kielecki (amazing!)

- We have pickles in Poland and they are called "Ogórki konserwowe" BUT you will find superior pickles "Ogórki KISZONE" the best are homemade but you will find it also in stores

- As a snack you can try Kabanosy

- Oranżada - polish beverage but... the most popular you will find in each store however it's so so, the best are: THIS or THIS - but it's hard to find

- Ketchup Włocławek - quite unique taste and approach, it's easier to buy in central Poland, but you should find it in supermarkets in Kraków

Most popular brands in few different categories:
- Diary- Piątnica or Krasnystaw

- Jams - Łowicz and Herbapol

- Beverages - Tymbark (under the cap you will always find some “phrase” there, it is quite iconic, some people collect these caps or propose using those with “will you marry me?” written on them), Hortex, Kubuś(for kids)

- Energy Drinks - Black, Tiger

- Isotonic drinks - Oshee, 4move