I think they'd say PU, UK, MSU .
LOL, I think we all know the answer:
If it were a poll that they knew Illini fans might see (like on a national board), we wouldn't get any votes. If it were a poll done of what they all really thought, I would think it'd be 1) UK, 2) PU, 3) UI with a clear dropoff after 3. A huge part of Indiana fans' completely forced and hilarious Napoleon complex is being absolutely adamant that Illinois is NOT a rival. I guess mingling with teams that have no National Championships (minus, ya know, Purdue, LOL) is just so below them*.
* (Who they think they are/want to be)