An archlich is presumably the highest ranking lich in the Scourge. Kel'Thuzad is the only known holder of this position so far.

In Wrath of the Lich King Edit

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The only evidence of this rank is that Boss 15 Kel'Thuzad drops an item, [Wand of the Archlich], in Naxxramas. By inference, that would make Kel'Thuzad, the Archlich.