For reference for people, I'm going to go through and post additional info about the games mentioned at the start:
- 2015 Wisconsin - Season opener, preseason #17 ranked, had gone 36-4 the previous season and lost in the NCAA championship game. That season, Wisconsin finished 22-13 and made it to the Sweet 16.
WIU had finished 8-20 the previous season and would finish 10-17 that season.
- 2004 Loyola - Loyola was 2-4 at that time, finished the season 13-17, not ranked during the season, and no postseason tournament. WIU went 11-17 that season. Game was at Western Hall.
- 2001 Drake - Drake was 2-1 at that time, finished the season 14-15, not ranked during the season, and no postseason tournament. WIU went 12-16 that season. Game was at Western Hall.
- 1997 Bradley - Bradley was 4-3 at the time, finished the season 15-14, not ranked during the season, and no postseason tournament. WIU went 16-11 that season.
- 1994 Dayton - Dayton was 1-0 at that time, finished the season 7-20, not ranked during the season, and no postseason tournament. WIU went 20-8 that season. Dayton was coming off a 6-21 season...I'm not even sure this would be considered an upset. They were a pretty bad mid-major at that point, and we were a pretty decent one.
- 1994 Northwestern - NW was 4-3 at that time, finished the season 5-22, not ranked during the season, and no postseason tournament. WIU went 20-8 that season.
- 1993 Dayton - Dayton was 1-2 at that time, finished the season 6-21, not ranked during the season, and no postseason tournament. WIU went 7-20 that season.
- 1990 Northwestern - NW was 1-0 at that time, finished the season 5-23, not ranked during the season, and no postseason tournament. WIU went 13-15 that season.
- 1989 Butler - Season opener, Butler was 11-17 the previous season, finished 6-22 that season, not ranked during the season, and no postseason tournament. WIU went 16-13 that season.
- 1989 DePaul - DePaul was 3-4 at that time, finished the season 20-15, not ranked during the season, went 2-1 in the NIT. WIU went 16-13 that season.
- 1987 UCF - Central Florida was 2-8 at that time, finished the season 12-15, not ranked during the season, and no postseason tournament. WIU went 12-16 that season.
- 1985 St. Louis - STL was 1-1 at that time, finished the season 18-12, not ranked during the season, and no postseason tournament. WIU went 13-15 that season.
- 1983 Cincinnati - Cincy was 1-3 at that time, finished the season 3-25, not ranked during the season, and no postseason tournament. WIU went 17-13 that season.
- 1981 Oklahoma - OK was 5-1 at that time, finished the season 22-11, not ranked during the season, went 3-1 in the NIT. WIU went 14-13 that season.
So, looking at those, wins over teams that would finish the season with a winning record: 2015 Wisconsin, 1997 Bradley, 1989 DePaul, 1985 St. Louis, 1981 Oklahoma.
Teams that played in a postseason tournament that year: 2015 Wisconsin, 1989 DePaul, 1981 Oklahoma.
I'd say those three games are probably pretty high on the list of biggest program wins.
I also went through and compared the SRS (
https://www.sports-reference.com/cbb/ab ... y.html#srs) of that year's WIU team and the opponent that we beat.
In 5 games, we had a higher SRS, which means that (using this simplified rating system) WIU was actually the better team that season: 1983 Cincinnati, 1987 Central Florida, 1989 Butler, 1994 Northwestern, 1994 Dayton.
Comparing those numbers, the biggest "upsets" (their team rated as significantly higher than ours) were (in order): 2015 Wisconsin, 1981 Oklahoma, 1997 Bradley, 1993 Dayton, 1989 DePaul.
The furthest back I was able to find RPI ratings was 1993, so of the 7 games since then that were on this list, we were listed as having a better RPI than both Dayton or Northwestern in 1994, we were within 90 spots of 1993 Dayton, 1997 Bradley, 2001 Drake, and 2004 Loyola (I'd consider that roughly in the same vicinity...upsets, but not huge upsets). Then you have 2015 Wisconsin, where we were a whopping 219 RPI positions away (43rd vs 262nd).
None of this takes into account things like the "condition" of the two programs, team's injuries or the impact that the win had on WIU's trajectory the rest of the season or the next year or two. Those things are hard enough to quantify with recent data (and nearly impossible to know if you're going back 20+ years).
So if you're asking about the biggest win in program history in terms of the importance of the program, I don't really know for sure. You can look at the following seasons after a big win and think that maybe some of the ones in the 93-94 range contributed to the success of the mid to late 90s (winning seasons from 94-95 through 98-99), even if we were actually favored in those games.
If you're asking about the biggest single win in terms of the biggest upset, I don't think there's any question that 2015 Wisconsin has got to be it, with probably 1981 Oklahoma coming in at #2. #3 is probably the 1997 Bradley game (a team led by future Leatherneck HC Jim Molinari).
EDIT - a bit more info. Found an article talking about the biggest college BBall upsets in terms of spread/moneyline, our win over Wisconsin was #2 (although there was no moneyline in the 1982 defeat of Virginia by NAIA Chaminade). We were listed with a +25.5 spread, which means UW-Madison was favored by ~25 points.
https://www.sportsinsights.com/blog/big ... st-decade/