Tails is the best Sonic character. He can be used in any story and not feel forced, his main ability is woven into his design, easily recognizable silhouette as well as a great and unique color scheme. His backstory and motivations are simple and don't tie him down. Gameplay wise he's pretty fun in 2D, haven't been able to play as him in 3D. He also just... fundamentally works. Characters like Shadow, Blaze and Silver, which also have backstories, can't work in some settings. The story often needs to accommodate these characters in some way. But Tails is so flexible he's the go to for the supporting character
His inventions and genius also serve him well. It can give the writer an easy excuse to give him new things to make and research, keeping things fresh similar to Eggman and his new robots. Of course it's not to the same extent as Eggman, but Tails' knowledge is also versatile.
I really can't think of any flaws narratively wise. He's a character that grows and evolves, helping others and being friendly, and that simplicity makes him endearing