It seems hard to pick one of Schilt, Aerts and Hoost, but I would lean towards Schilt as the greatest of all time. I think it's also worth mentioning that even though the glory days of kickboxing were in the 90s/00s, Rico Verhoeven's current streak of title defences is a remarkable achievement in kickboxing, if I'm not mistaken it's 10 title defences in a row, which none of the K-1 era guys did as far as I'm aware.
That being said, fighters used to fight a lot more frequently, and also fight in one night tournaments more often, so it's hard to compare across eras. Nevertheless, I think Verhoeven is up there in the conversation at this point.