There are tremendous amount of summer festivals in Japan, from small and local to big and famous ones, and hard to decide the best one!
Here are some famous ones:
Aomori Nebuta Festival (Aomori)
This should be one of the most famous summer festival in Japan. 3 million people visit this festival every year! The huge lantern float with neat decorations will sure be surprising.
Gion Festival (Kyoto)
The festival is one of the "Three Great Festival of Japan". It continues for a whole month! It's also one of the oldest festivals continuing from 9th Century.
Fukagawa Yawata Festival (Tokyo)
The festival of Fukagawa Shrine (located in the east area of Tokyo) held in every two years. Many stroll are out and traditonal Omikoshi will be out. You can feel how authentic festival is like.
Azabujuban Festival (Tokyo)
Held in Azabujuban area (near Roppongi), many people visit here in yukata. There are many food strolls out and those food strolls are operated by the restaurants in the Azabu Juban Shopping street. It is said to be the food quality in restaurants here are great, having international food because there are many embassies in the area. You can enjoy delicious, restaurant quality food in a food stroll price at this festival.
Festivals in Tohoku:
Festivals in Tokyo: