The best friend is usually my favorite character. Or, you know, the characters that play a major role but not the central character?
In the Study series by Maria V. Snyder, it was Ari and Janco.
In the Protector of the Small quartet by Tamora Pierce, it was Neal.
In the High Heels Mysteries by Gemma Halliday, it was Dana.
In the Dragon King trilogy by Stephen R. Lawhead, it was Toli.
I need to think up more books. All of these are series, and only one isn't a fantasy of some sort.
Oh, got one! In The Wedding Bargain by Victoria Alexander, it was Cynthia. That's not a fantasy, although it is apart of a romance series. Egh, close enough.
Ethel to Lucy. I Love Lucy.
Gerald to Arnold. Hey Arnold!
How could I forget the most famous best friend probably in all literature? Samwise Gamgee from Lord of the Rings by J. R. R. Tolkien.
The Best Friend is the character that, in time with providing comic relief, will make sure the goal gets reached even if the hero (or anti-hero) fails. They will see this through. They are loyal to a fault. In some cases, they're the voice of wisdom.
In some cases, a family member will serve the position as the Best Friend. Our protagonists would be pathetic people if they didn't have friends. Who would help them with their struggles? Their anxieties? And everything else that the creators of the story throw at them?