re: What's the most devastating regular season loss for each football program?

I would say the 2014 Egg Bowl. However, the playoff committee would have likely jumped Ohio State over us anyway after they blew out Wisconsin 59-0. So the best we would have had is the Orange Bowl, which is where we ended up.

Aside from that, we had losses against Bama in 1999 and Arky in 2000 which derailed any chances at winning the West in those years.

Oh, and I forgot that our loss to Florida in 1980 cost us a share of the SEC championship.