So I think we are basically in agreement that in our lifetimes, it's 1995. 1936 might be in the discussion, but there's really no fun in that comparison.
So let's do aside from 1995 (and ignoring the old teams none of us saw), what's the best Northwestern team?
1996 - Won a 2nd straight Big Ten title, going 7-1 in conference, and 9-3 overall, losing to Tennessee in the Citrus Bowl to finish #15. The OOC loss to Wake Forest doesn't look great. And the other two losses were to top 10 teams (PSU and Tennessee), but neither was close, losing by 25 to PSU and 20 to Tennessee. After a relatively easy 35-17 win over last place Indiana in their Big Ten opener, 5 of their other 6 Big Ten games were by 4 points or less. By 1 over #20 UM is fine, but by 2 over Minnesota, 4 over Wisconsin, 3 over Illinois and 3 over Purdue. None of those teams finished .500 in Big Ten play, going a combined 7-25. They avoided #2 Ohio State and a Michigan State team that finished over .500 in Big Ten play. Oddly, their best win (over #18 Iowa) was their biggest margin of victory, winning by 27 in Kinnick. Darnell Autry fell from 4th the previous year, to 7thin Heisman voting.
2000 - We already sort of discussed this one. Got blownout OOC by LaDanian Tomlinson and TCY. Won back to back road games against Wisconsin and MSU, who both turned out to not be nearly as good as they were thought to be at the time. Lost to Purdue, got gifted a win over Michigan by the officials, lost to a 3-9 Iowa team, then got stomped by Nebraska in the Alamo Bowl. They won a Big Ten title, so that's hard to ignore. But it feels like they backed into it out of a variety of factors (missed OSU, shouldn't have won the UM game, had Purdue get upset twice)
2012 - This was the team that really changed Fitz's narrative from "as good as NW could do" to "NW is lucky to have this guy." After five straight years of losing between 4-6 games, he went 10-3. It's still a tough team to gauge. They missed OSU, Wisconsin and Purdue, lost to PSU, Nebraska and Michigan, meaning they only played 4 bowl teams, and the only one they beat was a 6-6 MSU team, and they only did so by 3. But they did handle that schedule other than an opening week scare against Syracuse, winning the rest (other than the aforementioned MSU game) by more than 1 score. And they had every chance to beat both Nebraska and Michigan, and head to Indianapolis. Nebraska trailed all afternoon, but Taylor Martinez threw 2 TD passes in the final 5+ minutes to overcome a late 12 point deficit, and UM beat them in OT, after the NW defender let a WR get behind him for a 53 yard pass play with 0:18 left to set up a tying FG. Gator Bowl win was the program's first postseason win in over 60 years.
2015 - Opened the season with a 16-6 win over a Stanford team that wound up finishing as Rose Bowl champs, and ranked #3 in the nation. Probably kept Stanford from the CFP with that win. Started 5-0, and got up to #13, before getting crushed in back to back weeks by Iowa and Michigan by a combined 78-10. Won out, including a 13-7 win in Camp Randall, to reach #12 in the polls, before getting smoked 45-6 by Tennessee in the Outback Bowl. 10 win season is tough to ignore, but there was a clear top 4 in the Big Ten that year, and they avoided MSU and OSU, and got crushed by Iowa and Michigan, plus a pretty mediocre Tennessee team. The win over Stanford is as impressive a resume win as anyone had, but it was opening weekend, and that was certainly not the Stanford team that was rolling by the end of the year. Justin Jackson was a workhorse, and churned out 1,418 yards, on a career high carries, for a career low ypc, but he was the offense. Clayton Thorson gave them nothing as a freshman.
2017 - The first half Northwestern went 2-3, got crushed by Duke, Wisconsin and Penn State, and was lucky not to lose to Nevada. Second half Northwestern won 8 in a row, including over a ranked MSU team. After three OT wins in a row, they rolled Purdue, Minnesota and Illinois all by double digits, averaging a MOV of 28 over their final 3. Managed to pick up a bowl win over Kentucky despite losing their starting QB early in the game. Obviously the schedule eased up after Wisconsin/Penn State back to back, but Northwestern got clearly better too. Tough to say how much each was a factor. Common thread with all 5 of these...they avoided Ohio State, who they've only beaten once since 1971.