2012 wins for me. I remember it hitting 100F in 1988 but I feel like the beginning of Summer wasn't hot at all, maybe even cool? 1995 was crazy with how high the dewpoints were with the heat. However, I remember there being some good backdoor cold fronts which brought highs down into the 70s around here. I think we were really on the NE edge of the heat and Ontario actually had much cooler weather, hence the backdoor cold fronts and also the big derecho that July. 2012 was warm to hot already in May and seemed like it just never gave up until September, when it actually cooled way down the second half of that month.