Stortorget 3 (see markings at the map below)
This is probably the most visited patisserie in the Old Town. That is most likely due to their excellent homemade pastry, home baked bread, homemade cakes etc. But it also helps that they are located at the Main Square and have a nice outdoor area facing the square but an even nicer at the backyard (a very quiet and cosy place) and a rather nice interior as well. But it might also have something do with that they are a charity organisation where the profit goes to helping homeless people in Stockholm. Yes, I think you got it - It's a really friendly place.
If you like to buy some of their homemade items with you they have their own bakery shop next door.
By the way, did you know that they have their own church at the Main Square? It's called the Bun Church. It's located on top of the bakery shop. The name "Bun Church" comes from the fact that all homeless persons that attend a ceremony at the church will be given a cup of coffee and a cinnamon bun afterwards, for free. At Christmas Eve, Easter day and Midsummer Eve they get a full meal.