The two worst Ohio State football coaches in the last 60+ years were Earle Bruce and John Cooper. To be fair, Ohio State has only had five head coaches since Woody arrived in 1951 (not including interim coach, Luke Fickell) and the other two on the list are Jim Tressel and Urban Meyer. Earle and Coop were both fine coaches, but on a list as illustrious as HC at Ohio State, a few names inevitably have to occupy the lower spots on the totem pole. IMO, the coach on the bottom rung is Earle, not Coop.
The opposing argument - Earle over Coop - which I'll try to refute, is usually based on comparing raw numbers:
Earle Bruce: 0.755 win percentage; 5-4 v. Michigan; 5-3 in bowl games.
John Cooper: 0.715 win percentage; 2-10-1 [edit: corrected from 1-10-1, which was erroneous] v. Michigan; 3-8 in bowl games.
The above numbers are misleading, though. First, when Bruce arrived at Ohio State in 1979, the talent left behind by Woody was very strong. Earle's best year at Ohio State was his first year, 1979, playing almost exclusively with Woody's players. Earle's second best team might have been the 1980 team, with the key players again mostly having been Woody recruits. In 1980, Ohio State lost to #11 UCLA (Kenny Easley and Freeman McNeil), to #10 Michigan in a tight defensive struggle, and to an underrated #10 PSU (Todd Blackledge, Curt Warner, etc.). Thereafter, Earle struggled to recruit enough of the highest-caliber players that we expect at Ohio State, especially on the defensive side of the ball.
By the time Cooper arrived in 1988, the talent cupboard was pretty bare by Buckeye standards, prompting his infamous "too many slow white guys" quip. It took several years for Coop to rebuild the talent base. If we allow that Coop's first five years at Ohio State involved a heavy rebuilding project, whereas Bruce was set up for success from the get go, we see that Coop's post-1992 winning percentage was 0.773 (or 8th best in the nation during that period). Some fans believe that Coop kind of lost control of the program toward the end of his regime - which is the main reason he was fired - but otherwise the program was mostly on the upswing during Coop's reign.
Second, the Big Ten was probably stronger overall during Cooper's time at Ohio State compared to when Bruce was head coach. In the 1980s, the Big Ten was still kind of the "Big Two and Little Eight." Iowa under Hayden Fry had some big years, Purdue was good at times, etc. After Cooper took over, though, "parity" was in vogue. Programs that historically had been bottom-feeders either had a serious resurgence (e.g. Wisconsin, Illinois) or enjoyed several big years (e.g., Northwestern, Indiana). The overall quality of coaching continued to improve (e.g., Alvarez and, briefly, Saban at MSU). And, obviously, PSU joining the conference in 1993 also elevated the overall level of competition.
Finally, while there is no excusing Coop's 2-10-1 [edit: correction] record against Michigan, it was a relatively tough era in which to play Michigan. Whereas Michigan had the 12th best winning percentage (79-21-1) between 1979 and 1987, they had the 7th best winning percentage (121-33-4) between 1988 and 2000. Granted, Bruce is responsible for some of the losses that lowered Michigan's overall win percentage in the 1980s. But it was also because the Michigan program had several middling seasons (by Michigan standards) toward the latter part of Bo's great run at Michigan, although Bo did go out in style his last two years there.