Interesting that only one of the OP's top 4 make my list
1. 2019 Iowa: I could see arguments for #1 and #2 being switched on my list, but to me, there is a clear top 2. We had just beaten Penn State and had a perfect season on the line. The amount of National attention and local buy in we had was at the highest point I could remember. Whether you believe the team came out flat or we simply were outcoached early, we dug ourselves an awful early hole and then proceeded to be the better team for the final 2 1/2 quarters or so. Tyler Johnson was open and had a relatively easy go ahead TD hit him in the numbers on 4th down late. The sheer pain of letting Iowa (my most hated team) end our perfect season, makes this the worst loss in my lifetime as a fan. If we win this one, we clinch the West before we even play Wisconsin in the final week of the season. It's also two more weeks of being discussed as in contention for the CFP and two more weeks of great positive buzz for the Gophers on a National and local level. Instead, the Gophers are quickly removed from being Nationally relevant. To be clear, I am not saying the team makes the CFP, but at least a couple more weeks of being undefeated and having a guaranteed trip to Indy would have been enormous for this program. Winning this game also could have changed the result of the Wisconsin game with the Badgers no longer having chance to play for a West title and possibly PJ wouldn't have been so tight?
2. 2019 Wisconsin: Very similar to the Iowa game. Where Iowa ended our perfect season, our other rival came in to our house and took our trip to Indy. This was Gameday in Minnesota again and a chance to really cement a fanbase who had just seen their first significant win in the Bank's history a few weeks prior. Winning these types of games makes people fans for life and encourages them to bring their kids in to the fanbase. Losing this game in the first home game after beating Penn State (some fans were literally breaking down with happiness after that one) was such a gut punch to the fanbase. The way we lost this one while getting ahead quickly and then playing ultra conservative while Paul Chryst of all people was going for 4th downs and dialing up trick plays will be remembered forever.
3. 2022 Purdue: This one was just completely shocking to me. The Gophers had just played arguably their best game since 2019 in a win at MSU and National media types were calling them a huge favorite to win the West. I ALWAYS have my guard up when it comes to the Gophers and I didn't see this coming at all. I believe it was the same week of the season as the Bowling Green game the year before (I know they were both Homecoming), and in many ways the game was a carbon copy. The Gophers come out completely flat and Fleck panics in a one score game early in the first half by going for (and failing to convert) a 4th and 1 from deep in his own territory. We still win this game if MBS doesn't drop an easy TD or (likely) if Trickett doesn't miss a chip shot field goal. It's hard to see us not ending up in Indy if we simply win this game as like 10 point favorites at home. Just a horrific loss with the context of it being so similar to the Bowling Green game a year prior making it that much worse.
4. 2023 Northwestern: I could probably put 5-6 games here. The 2021 loss to Bowling Green was in the top 20 (maybe top 10?) for largest upsets by point spread in the 40 plus years we have data for that sort of thing. The 2021 loss to Illinois right after PJ got extended is another one. The 2022 loss to Iowa where the Gophers complete dominate the game but are so afraid to do anything else other than hand the ball to Mo that they find a way to choke away another B1G West title opportunity. The 2023 loss to Illinois could go here for the absurdity of allowing a backup QB to enter the game cold an go 85 yards in 35 seconds with no timeouts for a TD. The 2020 loss to Maryland as near 20 point favorites (almost the exact spread of the Michigan game this year for context) also deserves mention though because the season was awful it can't be in true contention. I give the Northwestern loss this spot for the sheer ineptitude of losing a 31-10 4th quarter lead to a team that hadn't won a B1G game in more than a year and would play a nailbiter with Howard a week or two after this debacle. Between the defense looking completely inept on every play to Fleck/the offense being completely scared to do anything more than essentially kneel on the ball to the decision to punt from the 37 to the punt gunner inexplicably lining himself up in the endzone, to the OT drop by Spann Ford followed by the immediate bite on the trick play after settling for the FG...it was like a blooper reel from a decade of bad football except it all happened in the same of a little over a quarter.
Best wins?
2019 Penn State
2019 Auburn
2018 Wisconsin- finally getting the Axe back
2021 Wisconsin- denying Wisconsin a B1G West title (bittersweet for me because it sent Iowa)
Not sure what #5 would be. Maybe one of the really impressive wins where we looked really good in '19 like at Purdue or home against Nebraska? Maybe beating Nebraska in '20 (biggest point spread upset PJ has had at Minnesota with a shorthanded team but the game didn't really have any implications). Maybe '22 Wisconsin in that it somewhat salvaged a decent season and allowed us to actually keep (defend?) the axe for the first time in forever?
The bad losses certainly far outnumber the good wins. How about Fleck runs the table (that means beats Ohio State) and puts the pressure on Iowa to go 2-1? It would be nice to be able to add a best win to the list in '23.