1979 undefeated regular season, lost Holiday Bowl 37-38 to Indiana. Finished year Rank 13/12. Ranked 11 at 6-0. Before bowl loss ranked 9
1984 National Champions, undefeated. Ranked #1 last 4 polls, both polls. Part of 25 games win streak
2001 started 12-0 reached 8/9 in polls and 12 in BCS standings. Injuries derailed the last 2 games. My favorite season (1979-1985 was before I was paying close attention). LUUUUKE.
2008 started 6-0, but lost 3 games. Peaked 7/8 in polls and BCS 14
2020 Started 9-0, lost to CC on 3 days prep on last second failure to gain a few extra inches. Peaked at 8 in both polls, 13 CFP rankings.

Started 5-0, but not 6-0
1981 Ranked 8 at 5-0, Finished 11-2
2021 started 5-0, peaked at 10 in both polls, 10 in CFP rankings.

Other great teams:
1983 Lost 1st game, but won 11 straight (peak ranking 7/8), plus undefeated 1984, plus won first game of 1985 for 25 game win streak
1996 lost 3rd game, but first team in country ever to win 14 games in a season at 14-1 with Cotton Bowl victory. Ranked 5 last 2 weeks both polls
2006 finished on 10 game win streak, 11-2. Finished 15/16 in polls and 20 in BCS
2007 won last 10 games, finished 11-2. 14/15 in polls and 17 BCS