Tommy McLean is the best in my lifetime, no doubt.
Brought us up, built a great team, won us the Cup and came as close as any modern day Motherwell manager is ever likely to come to winning the league (despite finishing 3rd).
Without Tommy, no way McLeish gets us to second place.
Favourable mentions to Craig Brown, who despite his Judas behaviour at the end, was a very good manager at the right time, and really should have had us through to the group stages in Europe.
McCall got us best of the rest 3 times and Champions League Football, but he did have a favourable budget and as others have said, was pretty shit in the cups.
No one has mentioned Robbo? 2 Cup Finals in one season is pretty special and if he hadnt come up against one of the best Celtic sides in a long time (twice) he might have an even greater legacy.
Worst? Has to be Malpas. The results were honking. The football was honking. Some the players he signed were honking, and he was boring as hell!