Well,i really dont know.
Both!
It depends on the person.

girls

just cuz boys are moody in my opinion (and I know it sounds like it should be the opposite)

i am a tom boy and guys are much easier to talk to

I am a girl who only has a couple guy friends when most of my friends are girs

*Growls.* What kind of question is that?! Anybody can be a great best friend! Stop being so genderist!

Im a gorl but some of my girlfriends get bitchy/moody/get pissed over small things but guys are not as moody

Girl i meant

it's mainly bias as im a girl but best friends are not only there to hang out with sometimes you need emotional support and though this might sound sexist it proven that girls are better at the emotional part of a friendship than boys are, e.g. if a girl gets dumped and her best friend (who in this scenario is a girl) brings the ice cream and the comfort but if a guy gets dumped his best friend (who in this scenario is a guy) tells him to shake it off that its a bummer and all but no need to dwell anyway all the friends that i've had that have been boys were more likely to take the p*** out of me when i was unhappy than be supportive,

Girls, just because I am one, and its easier. I have best friends of both though.

Guys are just nicer and easier to talk to. They judge less, and besides, how often do girls get in fights with their guy friends? Like never. But girls fight all the time.