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.