"Hotness" can vary from pepper to pepper. When we're talking about 1.5 million versus 2 million Scoville units, the pepper perceived as "hottest" could easily jump back and forth.
The Scoville Unit itself is only an approximation based on the reaction of tasters in a controlled tasting.
The Scoville scale is an empirical measurement dependent on the capsaicin sensitivity of testers and so is not a precise or accurate method to measure capsaicinoid concentration, however, capsaicin concentration can very roughly be estimated as ~18µM/SHU.
Personally I've used some extremely hot concoctions on my food. This one in particular, The Final Answer
, had the highest Scoville rating - that's hot.
In my opinion, the measurement should be not just of heat, but of "total combined discomfort". I have ingested less "hot" sauces that caused a lot more combined discomfort ~ mainly due to the viscosity of the capsaicin oils. If it's viscous, settle in for a lot of pain.
I have consumed habaneros, ghost chilis, thai chilis, Nagas, and a scorpion directly off of the plants. If you do this, avoid the seeds and the stem meat at all costs. But actually, I recommend not doing it at all. It hurts. I will say that as a class, habaneros are relatively mild, but I have had super-hot individuals. In fact, the odd super-hot habanero is probably the "hottest" thing I have ever eaten in terms of "total combined discomfort".