>>1211327571, Webisodes (2D), should be up on YT I think. Very loosely connected but all the girls are obvious archetypes so you can catch up easily. This has the original boxart design and probably the sexiest.
2, Movies in release order. These are mostly zero effort and used very cheap and dated 3D. The only point was to promote a new line of toys. Later ones got a slightly more budget and look more like a movie (like 13 Wishes and Boo York)
3, Reboot movies (Welcome to monster high + Electrified). The last and very short part of the saga for now. Basically the doll line was changed for the worse but at the same time it got movies with a proper budget. The cast in the movies and the origin was also changed. These designs are more cute than sexy.