Slim pickins that's for sure. 1984 was OK minus the bookend losses.

You obviously meant 1983. Illinois did something that year that no other team has ever done. They beat every other B1G team. That was during the brief 9-game league schedule era that coincided with the league only having 10 teams so Illinois played them all and went 9-0.
That much is great but . . .
- Their OOC was abysmal with a 10 point loss to Mizzou, a 10 point win over a horrible Stanford team, and a blowout RoseBowl loss to one of the worst RoseBowl Champions ever.
- They had a remarkably fortuitous H/A split. Of their five league road wins, only the one in Madison was over a team that finished above .500 and Wisconsin was 7-4 so not great. At home they got three 9-3 teams (Iowa, tOSU, M).
I think my starting point would be final AP ranking and frankly Illinois hasn't had many of those, only 13:
- #3 in 1963
- #4 in 1951
- #5 in 1946
- #7 in 1953
- #10 in 1989
- #10 in 1983
- #12 in 2001
- #13 in 1959
- #13 in 1950
- #15 in 1944
- #20 in 2007
- #24 in 1999
- #25 in 1990
1963:
Oddly, they lost at home to a bad Michigan team (M finished sub .500). They tied tOSU in Columbus, won the Rose Bowl, and finished 8-1-1 and #3. This is arguably their best-ever season as it is their highest final ranking and the loss to Michigan was close, 14-8.
1951:
They tied a mediocre Ohio State team (tOSU finished 4-3-2 in Woody's first year) in Columbus and won all the rest of their games including a blowout of Stanford in the Rose Bowl to finish 9-0-1. The final poll was still before bowls so it didn't reflect Illinois' blowout win over Stanford. This is probably their best-ever season.
1946:
They lost badly (26-6) to Notre Dame and lost by a TD at Indiana but won the rest to finish 8-2 and #5. Also, the final poll was before the bowls so it didn't reflect Illinois' blowout win over UCLA.
1953:
They tied a bad Nebraska team (Nebraska finished 3-6-1), got blown out by Wisconsin (34-7), and finished 7-1-1.
1989:
They got spanked in Boulder by a very good Colorado team (Colorado finished 11-1) and lost by two TD's at home to Michigan but won the rest of their games to finish 10-2 with a Citrus Bowl win over UVA.
1983:
This was the year the Badge displayed above.
That is all of their top-10 finishes. I think that 1951 was better than 1963 because they were undefeated in 1951. It would be tough to top a year that finished #4 (pre bowls) and 9-0-1. I don't think they'd get there this year without making is to the NC game.