The tops of these lists are pretty easy.
Swap if you'd like. I lived through the 90s and 2002 felt like the season we'd been so stinking close to in '95, '96, and '98. The win over TTUN was not merely nail-biting but therapeutic. 2002 also felt like a season-long climb with some crazy adventures along the way. 2014 OTOH cannot be matched from a post-season standpoint. From pitching a perfect 59-0 game in Indy to slaying the SEC gorilla in New Orleans, to finishing it against Oregon. And it only took 3 QBs through the season to do it! Take your pick, but short of being around for 1968, those are probably 1 and 2.
After that it gets tough and it comes to asking what's the milder flavor of bitter? A perfect regular season like 2006 + a NCG drubbing or a 2019 + a loss in a game where we outplayed the opposition? Or a 1996 where an utterly devastating loss exists, but the pain is mellowed by a nice bowl win. Maybe 2015 or 1998 type seasons where the loss doesn't come to TTUN rank higher for you? Or maybe perfection is your flavor albeit spiked with a bowl ban after a 12-0 season?
Anyway, where does 2019 fall for you?
Here's the rest of my top 15 for the last 30 years:
3. 1998 - simply because I loved that team. The Sugar Bowl win didn't add much (yay, arguably the best team in the nation was able to beat top 10-ish Texas A&M...), but the talent on that team was so complete and they provided one of just 2 wins over TTUN in the 1990s...by beating Tom Brady btw.
4. 2019 - it's odd to say this, but when it comes to perfect regular seasons, I almost prefer the anxieties of a 2002 regular season to the seeming ease of this one. Not a single team came within 2 scores and really not a single game until Clemson where the outcome ever felt the least bit in doubt. Had Clemson defeated LSU, this would jump 1998 for me knowing this team should have a NC, but even if I still can't accept the Clemson loss I can accept that LSU was really good and Justin Fields was playing hurt. I will say this...with a NC next year, this one moves up as it feels like the building block for something greater.
5. 1996 - so damn close. A win at ND and over PSU in back-to-back top 5 showdowns. That Rose Bowl win was epic. Just one thing missing.
6. 2005 - may seem odd to have a 2-loss season this high, but "The Catch" provided the first win at TTUN since 1987 in a great comeback and the Fiesta Bowl was great watching us race past ND at old Sun Devil Stadium in Tempe. The Texas loss was a dropped pass from being a win and an amazing game to watch. The second loss at PSU was tougher but that environment was insane. Overall this team ended on 2 high notes and left us feeling great going into 2006.
7. 1995 - another 2-loss campaign, but the Citrus Bowl felt meaningless after larger hopes just weeks before and it was a monsoon. The loss at TTUN was devastating. But taken as a whole, this team beat a top 25 opponent in 5 of their first 6 games! That included outstanding wins over ND and at PSU. Eddie's Heisman run...
8. 2006 - I never want to go into a coma, but between 11/18/06 and 1/8/07 would have been the time to do it, as I'd have been smiling in my sleep reliving that 1 vs 2 win and assuming the team finished their perfect season. In all seriousness, I have a hard time assessing 2006. That regular season was perfect, winning at 2) Texas and defeating 2) TTUN, going wire to wire as #1 into the NCG. The ending was a nightmare though, so it ranks 8 instead of 1.
9. 2018 - demolishing TTUN and their playoff hopes and winning a Rose Bowl in Urban's final game make up for Purdue and a lack of defense all year. The win at PSU was amazing as well.
10. 2016 - the wins at OU and in the 2 vs 3 OT thriller over TTUN were amazing. PSU literally stole a Big Ten title out from under us though and the CFP was nearly as awful as the 2006-07 NCG.
11. 2015 - expectations and this team never met, but I sure wish they had. In an alternate world this team beats Bama again in the NCG. In the real world, we have the most talented OSU roster ever, a mostly lackluster season followed by demolitions of TTUN and ND. While a 1998 type miss, this team just under-performed (or more accurately had an inept OC) for too much of the season.
12. 2012 - I'd expect most to have this perfect 12-0 season higher, but the post-season ban stung that much for me to knock it down. Even a Big Ten title over Nebraska would bump this up several spots, but perfect as the season was, it was a weird feeling after storming the field knowing it was done. One of the weakest schedules also factors in for me, lacking big-time wins.
13. 2017 - OU and Iowa stunk, but we won the Big Ten, had an awesome win over PSU, and beat our old Rose Bowl foe and Pac 12 champion, USC in a bowl.
14. 2007 - another that could be higher, but the Illinois loss and backing into the NCG was weird. Close to a NC, but unlike 1996, 1998, 2015, or 2019, not one of our teams that I felt "should have won it or had a shot".
15. I really can't pick between 2003 (unsatisfying but finished top 5 with a lot of top 25 games), 2009 (Rose Bowl win, but otherwise just an ok season), 1993 (start of the comeback of OSU football), and 1997 (came up short in every big game, but this was a talented team that honestly came a play at TTUN from having a 1-loss Rose Bowl season).