Oricon has posted the top-selling book lists in each category for the first half of 2015 (from November 17, 2014 to May 17, 2015). Neither One Piece or Attack on Titan, the most popular manga series during the period was Nakaba Suzuki's The Seven Deadly Sins, which sold 7,059,400 copies in total. Meanwhile, the No.1 series in the same period of the last year, Hajime Isayama's Attack on Titan dropped to 4th, losing a significant 40 percent of its sales, 8,342,268 to 5,047,725.  

Manga Top 10 Ranking (by series)

1. The Seven Deadly Sins (Nakaba Suzuki, Kodansha) - 7,059,400

2. One Piece (Eiichiro Oda, Shueisha) - 7,046,066

3. Assassination Classroom (Yusei Matsui, Shueisha) - 5,231,158

4. Attack on Titan (Hajime Isayama, Kodansha) - 5,047,725

5. Haikyu!! (Haruichi Furudate, Shueisha) - 3,588,366

6. Tokyo Ghoul (Sui Ishida, Shueisha) - 2,640,791

7. Naruto (Masashi Kishimoto, Shueisha) - 2,459,918

8. Ao Haru Ride (Io Sakisaka, Shueisha) - 2,326,507

9. Terra Formars (Yu Sasuga/Kenichi Tachibana, Shueisha) - 2,303,931

10. Yowamushi Pedal (Wataru Watanabe, Akita Shoten) - 1,956,019

Manga Top 25 Ranking (by volume)

1. One Piece vol.76 - 3,079,249

2. One Piece vol.77 - 2,735,955

3. Attack on Titan vol.15 - 1,831,800

4. Attack on Titan vol.16 - 1,561,770

5. Naruto vol. 72 - 1,204,59

6. Haikyu!! vol. 14 - 904,661

7. Ao Haru Ride vol. 12 - 903,947

8. Assassination Classroom vol.12 - 894,596

9. Tokyo Ghoul:re vol.1 - 884,010

10. Assassination Classroom vol.13 - 859,449

11. The Seven Deadly Sins vol.12 - 850,540

12. Kimi ni Todoke vol.23 - 834,523

13. Tokyo Ghoul:re vol.2 - 828,807

14. The Seven Deadly Sins vol.13 - 817,169

15. Sakamoto Desuga? vol.3 - 763,866

16. Kuroko's Basketball vol.30 - 744,719

17. The Seven Deadly Sins vol.14 - 721,444

18. Haikyu!! vol. 15 - 720,254

19. Assassination Classroom vol.14 - 709,375

20. Haikyu!! vol. 16 - 689,076

21. Black Butler vol. 20 - 676,717

22. Detective Conan vol.85 - 664,546

23. Saint Young Men vol.11 - 655,446

24. Blue Exorcist vol.14 - 650,266

25. Terra Formars vol. 12 - 648,048

via: Oricon Style