User blog comment:B214/Question & Answers 5/@comment-108.224.166.119-20150507020532/@comment-3965848-20150507060150

I don't think he has an official best friend;

He has known Matsuda and Motohama longer but he's not as close with them as before since getting involved in all the supernatural stuff and they keep wishing for his misfortune (maybe if they had girls of thier own they'd stop hating on him).

Vali, Sairaorg and Saji are more considered as his rival's.

There are other male friends he has such as Bikou, Arthur, Loup Garou ect, but they are not as close to him like everyone else.

I think at some point Kiba stated he stated he was Issei's best friend but Issei has never stated this himself.