What would be next after minor, major, ....?

I'm looking for a word that would describe a third category following a "minor" and "major" category. (For compatibility reasons it won't do to insert a middle category of "moderate" or the like.) Example usage:

After fighting past several minor encounters, and a few major encounters, the adventurers finally made contact with the _____ encounter.

Solution 1:

If it's implied that there are no further obstacles - final.

If it's implied that there's no greater obstacle - supreme.

If it's implied that there's no passing the obstacle - overwhelming.

If it's required to keep some kind of hierarchy - capital.

Solution 2:

The word supreme is a good suggestion. You could also consider using ultimate:

greatest or most extreme

Final could be a candidate, too:

coming at the end : being the last in a series, process, or progress

[Merriam-Webster]