Gyoza Mania is one of those places I would confidently recommend to a friend.
It is only a short walk from Shinagawa Station, so it is very easy to add to your Tokyo itinerary.
The restaurant is known for its gyoza, which were recognized by the Michelin Guide, and the food is honestly delicious. This is not a touristy place created for social media - most of the guests are local office workers.
You can call in advance and reserve a table, which is quite useful in Tokyo. However, even without a reservation, the queue usually moves quickly, and I normally would not expect to wait much longer than around ten minutes.
In the evening, the atmosphere becomes louder and more energetic as Japanese workers come here to eat, drink and relax after work. It may feel a little busy, but that is part of what makes the experience so authentic.
For me, this was one of the most memorable local food discoveries in Tokyo: affordable, full of character and genuinely popular with people who live nearby.