the whole “somebody killed my best friend” “i lost my best friend” thing is like. well. it’s like. idk. sure it could mean, for some people, that crowley doesn’t care for aziraphale in a romantic way. and like, that’s valid i guess? but i remember asking my mom if she was in love with my dad, and she always gave the same answer: “he’s my best friend.” that’s how she spoke about her marriage. and crowley canonically listens to “you’re my best friend,” when he thinks about aziraphale, and that’s a song about marriage, too. and its lyrics describe their relationship! crowley, alone, screams for aziraphale while the lyrics “i know i’ll never be lonely; you’re my only one" surround him; perfect irony.
i also sort of feel like, in hell, having a best friend would be considered worse than having a lover. in heaven, it might be the reverse. uriel says “your boyfriend in the dark glasses” with the same distaste as hastur says “your best friend aziraphale,” y’know? and wouldn’t having a “best friend” be a shameful thing in hell, where no-one trusts anyone despite knowing them for centuries? crowley saying aziraphale is his “best friend” is an admission that he’s not trying to tempt him; no, he just likes him. he loves him. and “best friend” is the most unselfish way he can think of to say that.