Considering I don't have any official "best friends", I'll just talk about the friends I have who are... the best.
There are two who I consider myself to be closest to. One of them is a lot like me, and in general we have a lot of the same ideas and outlooks on things. We have a lot of the same humor and we basically grew up from K4 on together, so I've just always known her. We like a lot of the same music too, although she's more into movies, and neither of us watch a lot of TV.
My other best friend is probably a little more different. We have the same humor, as well as *some* of the same taste in music. I don't even know about movies and TV. But we tend to share overall opinions on things too. I mean if you look at her, she's the exact picture of the girls I usually dislike. I guess part of it is she doesn't go to my school so it's always exciting to see her (like my other best friend), but still, she looks just like a lot of the "pretty girls" at my school - really thin, some A&F/Hollister clothes, flats, perfect hair and skin... but I don't know, we just get along well.
As for my newer friends who I met this year, we actually probably have a little more in common, although we're not as close. We like a lot of the same music, have a lot of the same humor, tend to look at things and people the same way, and have the same thoughts on school (both academics and the school and whatnot itself).
As for relationships, none of my new friends who could be potential best friends have ever had a relationship, so... but they tend to think about guys the same way I do. Like, it'd be nice, but we're not going to start flirting and being over-the-top sluts just to say we have a boyfriend. My first "best friend" had a boyfriend for a looong time but they broke up in February, and my other best friend got a boyfriend just a couple weeks after my other friend broke up with hers.
And religion. My best friends and a few of my new friends are Christian like me, but a few are agnostic. We don't discuss it much or get into debates or anything, so I think we're all fine with each other on that.