My fiance passed away he raised my kids like he was there father how would i phrase that on the headstone how would i word that on the headstone do i put fiance or is there another word i can use?

You might consider using the term 'life partner', 'husband-to-be', or simply 'partner' rather than fiance; I can see why you could feel 'fiance' sounds a bit cold and formal.

Or you could decide there's no need to describe or define your late fiance in those sort of terms, but simply use his name and say that he's missed terribly by (your name) and (your childrens' names) and will live always in your minds and hearts, or something along those lines.

For your children, you could say what you've said in your question, that your partner raised them as if he were their true father; maybe you could get some input from your kids on this, depending on their ages, of course.

One comment, though, if your childrens' father is still alive and plays a caring role in their lives, you will need to choose the Wording very carefully so as not to cause pain to your childrens' natural father and to your children themselves.

Otherwise, the main people to be considered are you, your lost partner, your children, and, of course, his own close family, who might wish to add their own Words, bearing in mind they cannot speak for you: only you can do that.