Rice Krispies used to be my daughter’s worst, just plain with a little bit of milk and absolutely no added sugar, and they would immediately launch her blood sugars into orbit! We used to try to do the insulin bolus a bit earlier to minimise the effect, but often forgot or didn’t have time. White bagels were pretty bad too, I seem to remember that they were worse than other bread.
At the other extreme, a bowl of porridge was so slow burn that we had to do at least a 90 minute extended bolus to avoid her going hypo. And occasionally had the same problem with chocolate, specifically Wispa bars for some reason!
She rarely eats breakfast now and I don’t have anything to do with it any more, so the situation is probably different these days!