Final answer:
The biggest pop group in Japanese pop music history is considered to be AKB48, known for its large all-female membership, innovative fan engagement methods, and significant commercial success.
Step-by-step explanation:
The biggest pop group in Japanese pop music history is considered to be AKB48. AKB48 is a highly successful all-female idol group founded in Akihabara, Tokyo, in 2005 by producer Yasushi Akimoto. The group operates on a unique concept of 'idols you can meet', which allows fans to interact with members through handshake events and theater performances. Their commercial success is evident through their record-breaking singles and albums and their significant cultural impact in Japan. Additionally, AKB48 has spawned numerous sister groups in various parts of Japan and overseas, further solidifying their massive influence in the J-pop industry. While there are other contenders, such as Arashi and SMAP, AKB48's sheer size, innovative fan engagement approach, and sales records give them a remarkable place in the history of Japanese pop.