Was Souness that central to the team. Don't forget that we reached the european cup final the year after he left for Sampdoria and followed that up by winning the double in '86.
Personally, i'd go for Hughes. As captain from '73 to '79 we had tremendous success. He captained league championship, European cup, UEFA cup and F.A. cup winning teams. His enthusiasm on the pitch was infectious, as seen in his reactions to scoring. Also, leading the club to winning back to back european cups adds a few bonus points.