Depends what you mean by "best"? New steel mags are generally the best, but you may get equal performance out of much cheaper surplus aluminum mags.
https://www.hkspecialiststore.com/daily-deals
https://www.hkparts.net/shop/pc/HK-91-G3-Parts-c9.htm
https://www.hkspecialiststore.com/daily-deals
https://www.hkparts.net/shop/pc/HK-91-G3-Parts-c9.htm