Top 3 are easy for me:
1. Wullie Pettigrew
2. Bobby Graham
3. Tommy Coyne
Then
4. Higdon (albeit just for his record breaking season)
Number 5 is where it gets even harder. Was a big fan of Porter, but I'd go for McLair. Finding his feet in his first season and 16 goals for 37 games in his second season - not to mention his goals defeating the OF in consecutive games at Fir Park. More importantly, though, was you could see he was going to be a player.
Although I wouldn't argue if swapped 4 and 5. (In fact I think I would swap them).