There's a revolving sushi place (don't remember the name) in Tokyo under the shinkansen tracks in Hibiya/Ginza across the street from the Yurakucho Mullion building that's pretty good and their prices are reasonable.
If you live in Shimokitazawa, why don't you try Midori zushi in Umegaoka?
It's only two stations away by Odakyu line.
It's very close to the station and you cannot miss it because there's alwasy a long queue.
I could not find a website in English on this one but info on its branch in Shibuya: Midori-zushi: Shibuya
They have two restaurants in Umegaoka, and one take-away shop.
This is the main one which is only 1 min walk from Umegaoka station. 梅丘本館｜梅丘寿司の美登利総本店