My most utilized feature in iRacing? Well, it sure isn't the winner's circle, assuming they even have one. In my entire iRacing experience, I think I have won a grand total of two races, both in the Rookie classes.
No, my most utilized feature is actually one that has received scant attention from the dev team throughout the lengthy history of iRacing. Perhaps they simply ran out of other things to do, or maybe they felt some pressure from other racing sims that have done a far better job if it. The reasons don't matter - the fact is, iRacing is finally focusing some attention on an aspect of motor racing that I encounter on nearly every race: damage modeling.
Take my word for it - this is a significant undertaking. It is often the case that its longevity works against iRacing. Newer titles are (obviously) often built on newer code platforms that are flexible enough to allow for fancier modeling of physics and the like. iRacing has presumably has tons of legacy code that doesn't provide for more modern needs. What may sound like a simple enhancement may actually require a massive overhaul of underlying systems.
Case in point: iRacing has put out a video wherein various developers share the challenges inherent in such an in-depth change to an aging code base.
I found it utterly intriguing. Check it out and see if you do too.