I made myself some spicy chicken and macaroni soup, and put it in a bowl while it was still boiling.
I put a thermometer in it as it cooled and I started to eat.
I figured the soup was a good thing to test as I could take big bites that included liquid and solids that required chewing.
I took a bite every minute or so and noted the temperature, and I repeated with boiling soup after it had reached "too cool to enjoy".
At temperatures over 190F (87.8C)
I couldn't put it in my mouth without giving it time to cool on the spoon and "blowing on it".
At 180F (82.2C)
I was still "blowing on it", but not in an unpleasant way for the first bites of soup.
I would consider that to be optimum serving temperature for this kind of soup (see quote concerning Chinese noodle soup below).
At 170F
(76.7C)
I could put the bite straight into my mouth without pausing or "blowing on it" but it was still a bit hot, if I were eating without paying attention to the temperature, I would have "blown on it" At 160F (71.1C)
I can and did eat comfortably.
It was just pleasantly hot.
At 150F (65.5C)
it was still pleasantly hot.
At 145F (62.7C)
It was still quite warm, pleasant for soup.
At 140F (60C)
it was still fine, but not hot at all.
At 135F (57.2C)
it was still OK, but definitely getting on the cool side for soup.
At 130F (54.4C)
I was glad to be almost done.
At 125F
(51.7C)
I was thinking seriously about the microwave.
At 120F (48.9C)
It was too cool to enjoy this particular soup.
Obviously, different foods are going to have different optimal temperatures.
I wouldn't want steak to have ever hit over 130F (45.4C).
Also, consider that I have been cooking for 35 years.
I am accustomed to sticking a spoon into something boiling on the stove in order to taste it.
Consider too that there is (at least there is for me) something fun and vaguely comforting about soup that's still a bit too hot to eat.
Some of us blow on it, noodles are often slurped for that reason:
Slurping is de rigueur among the Chinese, for practical reasons.
“It’s a way of introducing cool air into your mouth to cool off the noodles and the broth if it’s a noodle soup. …
Chinese food is generally served hot, often served piping hot.”
- Chow.com
So, I don't think there is really such a thing as too hot to serve soup to "consenting adults".
Other types of food are cooled or rested before serving for other reasons.
Meat should always be rested to allow the fibers to relax so that the juices stay in the meat instead of running all over the plate.
Cheesy things like pizza are easier to cut and serve after they have cooled a bit.
Some things are just (subjectively) better after cooling (I prefer fried chicken barely warm, for instance).
So, that's my $.02 on the subject, the answer of "best temperature" is largely subjective.
cooking.stackexchange.com/users/20183/Jolenealaska
This most definitely is a food safety question.
I work for a school district, and we have had several instances of small children burning themselves on the soup.
We needed to write the proper serving temperature into our HACCP recipe to ensure that this would not happen again.
We did some testing ourselves and found that the 145-155 temperature range was optimal for best quality and safe transport on a lunch tray.
cooking.stackexchange.com/users/39171/Maylynn
I worked under a chef in Seattle for about six months.
He tended to recommend serving at about 150 degrees Fahrenheit.
cooking.stackexchange.com/users/52584/Matthew
The soup I ate tonight was too hot until it cooled down to 140 degrees.
At 145 degrees I had to blow on it first.
cooking.stackexchange.com/users/32763/Ron
You can order starbucks lattes at specific temperatures.
I find that 140 is about the hottest I can handle without having to wait, but I also have a sensitive mouth.
cooking.stackexchange.com/users/52426/Daniel
Thanks for the well considered answers- apart from the comments above, please consider the following which I have observed in forty years of restaurant ownership: Older people definitely require hotter food.
The temperature that a meal leaves the kitchen is way different to that of the last morsel eaten.
I hat heavy plate is a great help here.
Certain foods which have a large surface area and small mass (like grilled calamari)
cool very quickly.
In sous vide cooking, items like trout or rare meats which are cooked at about 52 deg C (125 deg F) are too cold to serve.
They will need searing or a blast of higher temperature to elevate them to the optimal eating temperature.
If the first few mouthfuls are pleasantly warm, the eater will seldom be unhappy with the cooler remnants.
Happy cooking Chef Nic
cooking.stackexchange.com/users/55505/Nicolas Proxenos
Well in the past year or so, I heat soup to 170F. Two reasons, one this temperature is about the temperature, once plated, that the soup is edible immediately, never burn your mouth again.
I hate to wait.
Second, per the USDA, at 170F, virtually all pathogens are goners.
Third, this temperature will destroy less nutrients then heating to boiling, a bonus.
Knowing this, I would think that restaurants that are concerned with table turnover should bring soup out at this temperature, this would allow people to eat quite hot soup, without waiting, and decrease the time they are at the table.
Just a thought.
cooking.stackexchange.com/users/61101/Douglas
I'm trying to get a temperature range for knowing when food is too hot to eat.  Most of what I am finding is temperature ranges for food to kill bacteria instead.  For example, if I'm serving a hot drink or a soup/stew which was recently simmering or boiling, what temperature should I let it cool to before serving?
What is the temperature range for food not to burn your mouth?
cooking.stackexchange.com
<temperature>
cooking.stackexchange.com/questions/46427
cooking.stackexchange.com/users/26561/RICARDO HOLMES
While 170F is the temperature at which virtually all pathogens are destroyed, 140F is about the temperature where it becomes cool enough to eat. For this reason, 155F may be a good temperature when poured into bowls on serving trays, to allow it to be pleasantly hot when it arrives at the table. 140 to 155 seems to be an acceptable temperature for eating food. However, it needs to be heated above 170 to kill pathogens.
Soup can never be too hot for adults. Other foods are cooled before serving for various reasons. Between 140 and 155 seems to be an acceptable temperature for eating food. However at 170 all pathogens are killed.