For Sushi, take the little restaurant opposite Skanstorget. Good food for a good price!
If you are looking for top-end sushi, and a higher prize level, I can totally recommend the resturant Koto on Viktoriagatan. they have some really good sushi, and the atmosphere is nice too. Be sure to wear your best socks should you end up in the japanese style section having to remove your shoes...!
There is also a smaller sushi resturant downtown at the pedestrian street opposite the Cathedral, can't remember its name, they have good o-bentos (lunch/dinner takeaways).
Favorite Dish: Nigiri sushi is always my favourite rather than the makis. However, sashimi is great too...!