The 2002 win at Michigan was better in my opinion. Now, I know the "freshness" of this OSU game matters a lot to people and hell many posters here probably don't even recall that 2002 season (how could you unless you're over the age of 22 or so) but going into the Big House and whipping them 34-9 was unheard of back then for a Hawkeye team. Much like hanging 55 on OSU, I know. Some of this comes down to opinion and recency bias. You know, I have no memory of that 1990 game at Illinois, but many fans who do believe it was just as special. Clearly I can't comment on anything pre that win so therefore I don't know all of the 1980s stuff with beating Nebraska, etc.
But for me, in my lifetime, I rank them as follows:
1) At Michigan 2002
2) OSU 2017
3) I'm not even sure what comes next. Wisconsin 2004, At PSU 2009, MSU 2010, At Minnesota 2008, Orange Bowl vs GA Tech, ???