I'd be genuinely shocked if Sakuya wasn't the character introducing the series with.
The only characters I feel they can start off with that give a good feeling of the verse, while also not being TOO COMPLICATED to introduce the series with, would be Reimu, Marisa, and Sakuya.
Marisa kind of has the issue of being matchup soup, she has a million matchups but not one particularly more popular than the rest. My pushing for Marisa vs Yuffie Kisaragi or Rose Lalonde hasn't spread wide enough YET to net her much. She also is no longer the LEAST complex or least haxxed character in the franchise, due to 18.5 making Ability Cards standard to her equipment set, which gives her more haxx than a lot of characters.
Reimu is a more likely story, cause a lot of her MUs are much more popular, and her being the main character is always a solid introduction. Sailor Mars, Arle Nadja, and a handful of others are all solid opponents for her.
Sakuya on the other hand, introduces the series well, has a relatively simple scaling chain and feats list comparative to other characters, has a very interesting and well designed power-set, and her major, most popular matchups in Hit and Homura are generally from very, very popular franchises that she could ride alongside to guarantee a pretty good viewership on her episode.
Other characters such as Cirno, while definitely a good candidate with good matchups, would require too much in verse knowledge about the functions of Touhou's systems and specific physiology to understand the complications with that MU, alongside her being drastically weaker than most other characters giving a poor introduction to the verse's scaling. Remilia, Flandre, Youmu, Yukari, and Kasen are also all options that have a good MUs, but also struggle with the issue of being too complex as a first introduction. God forbid, imaging trying to introduce Yukari as the first Touhou rep? With all her abilities?