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
WotLK This section concerns content exclusive to Wrath of the Lich King.
The only evidence of this rank is that Boss Kel'Thuzad drops an item, [Wand of the Archlich], in Naxxramas. By inference, that would make Kel'Thuzad, the Archlich.