Pretty easy for me, even if you split it into UM/MSU fan hoods. 1997 for Michigan, 2013 for Michigan State.
So, let me take those off the table, because they are so obvious.
For Michigan, which would be 1984-2004 (although 1990 was the first I remember), I think it's a tossup between 1991 and 1999. 1999 had the back to back losses to MSU and Illinois in the middle, but recovered and beat Alabama in OT in the Orange Bowl. But I'm going to lean 1991. They went 10-2, and both losses were ugly, but losing ugly to Florida State and Washington in 1991 was not the worst thing. It was my first UM Rose Bowl experience, and I remember after Desmond Howard's Notre Dame catch, we stopped playing catch in the backyard, we started trying to exclusively throw diving catches to each other after school.
MSU, being 2005 to present, 2021 actually absolutely has to be in the hunt. But, as fun as Kenneth Walker was, there's just a different feel to having a guy on campus for a year, and then leaving. 2015 resulted in a CFP, but that year was more of a walking heart attack than anything. Beat Oregon by 3 on a late 4th down overthrow; led Central Michigan by 6 with 5 minutes left; beat Purdue by 3; beat Rutgers by 7; beat Michigan on the trouble with the snap play; trailed Indiana in the 4th quarter; lost late at Nebraska; beat OSU on a walk off field goal; beat Iowa in the CCG in the final minute. I think it's 2010. It seemed like Dantonio was building towards something. He ended MSU's 4 year bowlless drought with a season ending comeback win against Penn State in 2007; then moved up to a NYD Bowl in 2008; but the wheels sort of fell off in 2009; with bad losses to CMU and Minnesota, and only getting bowl eligible with a comeback win at Purdue. But then 2010 started with low expectations, until they got the Little Giants OT win over Notre Dame. Two weeks later they upset Wisconsin, with Dantonio at home recovering from a heart attack, and followed that up with a blowout win at Michigan. They ran into a buzzsaw at Iowa, but needing a road win in Happy Valley in the final week to win their first Big Ten title in 20 years, they delivered.