Sora and Taichi
Taichi is one of her closest friends for many reasons but one of the primary reasons is she rarely ever feels the need to put on any sort of pretense around him. Often with her friends she plays mediator or maintains her own emotions silently in order to keep a situation from getting overly complicated. It didn't take long for Taichi to tear down that pretense around her.
When she's around Taichi she feels no need to hide her anger, sadness, or happiness. Although this often leads to friction between the two of them because Taichi is equally upfront about his emotions, they become all the closer for it.
On the other hand, when someone is hiding their emotions, Taichi is not terribly talented at noticing. He doesn't really realize or notice that Sora feels suppressed and when he does he just thinks she's depressed about something. As he's not talented at comforting someone when they're upset he tends to give ill thought out comfort. Either pushing her to talk about it or giving meaningless advice such as 'feel better'. He does, however, mean all of this with the earnest intention of helping. He just kinda really sucks at it. His most successful comforting is when he'll decide Sora is far too gloomy and they're gonna go on a mini-adventure.
These adventures range anything from hitting up an amusement park to exploring an unknown part of the city, occasionally they're as simple as seeking out the tastiest ice-cream in the neighborhood.
While these silly adventures rarely actually solve her problems, she does feel a little better after them.
If she were to name a best friend, Piyomon would always top the list, but with the exclusion of digimon Taichi is definitely her first thought.
