You and I are obviously of a similar vintage.
I would add T.C. (M'Lord) to your list of players I have seen personally. Cooper, Coyne & Graham all came to the club as established players, and without discounting their abilities, we didn't see the best of them at Fir Park. McFadden moved on before we saw the best of him but 48 caps and 16 goals in a pretty poor Scotland set up (with managers like Levein ) puts him right up there. Willie P also moved on, watching him in a Heart's strip was heart-breaking. So for me it would be Joe Wark, it still annoys me that he didn't get the international recognition he deserved.
That all said, I can't offer an opinion on those I didn't see personally thus making this an impossible debate, an interesting one though.