There are other rivalries which are noteworthy, like Florida-Florida State, Alabama-Auburn, USC-UCLA, Army-Navy, and others, but none combine the history (almost 100 meetings) and the annual significance (at least 1 team in the final top 10 every year since 1987, plus only 5 times since 1967 has the the OSU/Michigan game had no impact on the championship of the Big Ten. One team or both either finished first or were first or tied going into the game every year but 1999, 1994, 1987, 1985, and 1983. Plus, over those years, only once has either team had a losing record, OSU in 1988.) No other rivalry can claim this.
One thing fans of other rivalries always cite as a reason this one isn't the best is that it is rather uneven. Michigan does lead the series 55-35-6, but the Wolverines lead 13-0-2 before the 1919 game, and 19-3-2 before the 1928 game. Ohio State's only wins came during the 1919-1921 games. Taking out those first 24 games, the series is Michigan 36, Ohio State 32, 4 ties. It has remained pretty even throughout the time since 1928, when OSU finally started fielding nationally recognized teams.
Michigan has won 795 games in its history (1st overall in NCAA history), and Ohio State has won 717 (7th overall). Ohio State has had 6 Heisman trophy winners, Michigan 3. Between them, the Bucks and Wolves went to 13 consecutive Rose Bowls between 1968-1980 (1969-1981 Rose Bowls). Since 1967, only once (1987) has neither Ohio State nor Michigan finished either first of second in the Big Ten.
How's THIS for amazing? Since World War II (1945), Ohio State has won 421 games (421-144-20). Michigan has won 423! (423-154-15). Can't get much closer than that! Ohio State is 6th overall during that span, while Michigan is tied for 7th with Tennessee and Notre Dame. Since 1960, OSU has a (326-99-10) record-- 4th overall. UM's record is (324-109-10)-- 5th overall. Since 1989, OSU and Michigan have 93 wins apiece. Both are 7th over that span. Pick any period of time between 1900 and 1998 and Ohio State and Michigan are both in the top 10 overall of wins. Amazing. Compare the 20th century win total of OSU and Michigan (671+ 695= 1366) to those of Florida and Florida State (545+ 369= 914) or Alabama and Auburn (713+ 579= 1292) or Army and Navy (579+ 506= 1085), and tell me what the greatest rivalry of the 20th century is.