We can't
I am 38 - in the fandom I remember?
McNamara - bad
Morgan - good with the media, obviously would need help in hyper-information age ... but overall I liked him
Hobson - let's move on
Kennedy - could have been the best guy of them all, but really believed he was a great manager, which was the downfall
Williams - really good in his way
Kerrigan - Kennedy without the experience or ability to manage
Little - in some ways the right manager at the right time after the Kerrigan disaster. But ultimately the Pedro decision was unforgiveable, and he never was able to fully grasp the notion of the committee bullpen. On one level a bit underrated by Sox fans, but his biggest sin was a mortal one.
Francona - One of only two legitimate choices for the best Sox manager since 1967. Time will put him very much in the Anderson, Alston, Weaver, Herzog, Bochy level of the discussion. (I guess LaRussa - but he makes me angry). If he were your boss in your actual job (assuming he was qualified and trained) - he would be one of the best bosses you ever had.
Valentine - probably a bit underrated by his time here. But he put very little effort into this, while retaining all the ego.
Farrell - A solidly above average manager by the sorts of things you need a manager to do now. But there are definite holes, and tactically he could be better - but there are distinctly worse guys out there. I'd put him in the middle third of managers today.