Just occurred to me that there might not be one
Saiyan saga has kaioken which isn't technically a form but may as well be Frieza saga has super Saiyan, cell saga has super Saiyan grades and super Saiyan 2, buu has super Saiyan 3, bog has god, then blue, blue evolved and ui
Am I missing one?
I think at least in the anime, kaioken serves the exact same purpose as forms of just granting Goku a massive power boost to overcome a strong enemy and much like forms is more or less forgotten about once a newer shinier one comes around (until ssb kaioken much later on anyway)
To each there own but Kaioken didn't defeat Vegeta and Gokeu basically went lol SS I win to Friesa… Though it was great in the moment.