By that, I mean that the temperature strays the least from comfortable levels (60 to 80 fahrenheit) throughout the year.
The Hawaiian Islands would be my guess.
Costal Northern California
http://www.wrh.noaa.gov/eka/climate/summary.php
“Temperatures are quite moderate, and the annual range is one of the smallest in the lower 48 states. The record high in Eureka is 87F while the record low is 20F. During a typical year, the colder lows are in the mid 30s and the warmer highs will reach the mid 70s.”
San Diego has just about the best weather in the USA.
ETA- here are the averages, with the highs and lows being within 20 degrees or less of each other.
More specifically in coastal Northern California, San Francisco lows are typically in the mid-40’s in the winter, the highs in winter are usually in the low-mid 50’s, and there are maybe 3 or 4 days a year where the high is over 80. Nobody has air conditioning in their home, and we typically don’t use our furnace for a good 6 months a year (roughly May-October).
Roddy
I grew up in San Luis Obispo, CA (central coast) and the temperatures were extremely moderate.
In general, anything close to an ocean and at mid latitudes will be much more moderate than something inland. California is so moderate because the ocean water is very cold (50-60 degrees), but the air is usually warm.
Seattle’s temperatures are very moderated by this ocean effect, but only I seem to call 40-60 F moderate. 
When I was there it felt a trifle warm to me.
Mind you serves me right for being British.
Is extreme moderation possible?
Hopefully this won’t be much of a hijack, but I was pondering something similar lately, but thinking of the weather as a whole (and other potential natural disasters), and wondering where the most “calm” place to live was.
OK, time for too much of a peek inside my brain…I was actually wondering where I would live if I woke up one day and found myself the only human left in the world (give me a break, it was a very long car ride by myself). I was trying to think of a location that not only had the mildest temperatures, but also the least likelihood of a large-scale catastrophic event. The Atlantic coast is prone to hurricanes, the midwest to tornadoes, the west coast to earthquakes, etc.
I was wondering about eastern Tennessee/northern Georgia. I have not lived there, but from traveling through a number of times, that part of the country always struck me as pretty mild-mannered, climactically speaking. I assume there’s some risk of tornadoes, but nothing like you get in the plains, and there are probably some nasty thunderstorms from time to time, but I don’t remember ever hearing of anything that seemed too terrible.
Thoughts? Maybe this is more of an MPSIMS or GD question, since it’s pretty opinion-based. If so, I apologize for the derailment.
The last time I looked at one of those “50 best places to live in the US” guides, it confirmed that San Diego had the least temperature variation of all the major cities in the continental US. If you wish to include Hawaii, Honolulu has even less.
Sure. You can have too much moderation just like anything else.
I have often wondered about the weather of Northern Arizona … say around Flagstaff. Seems to be far enough north and at a high enough elevation to avoid desert heat, but Southerly enough and close enough to desert to avoid truly cold weather.
No. Elevation is a crucial factor. Flagstaff doesn’t get bitter horrible winters, but it gets cold and they get snow. It has a definite four season climate:
http://www.climate-zone.com/climate/united-states/arizona/flagstaff/
Note 209 days annually with a min temp below freezing, although very few days above 90. Flagstaff always struck me as a nice place, but the climate isn’t mild. It isn’t Fargo, ND, but it isn’t mild.
Another vote for coastal San Diego. (It gets hotter inland.) No tornadoes, no typhoons.
Just wildfires and earthquakes.
I think most of coastal Hawaii would win out over California. The Hawaiian mountains can be surprisingly cold, though. And coastal California is apparently more variable than you’d expect from the stats. I had a discussion about this on a gardening forum - apparently even within San Diego, you can go from extremely moderate (there’s that phrase again) on the coastal side, to very hot (in the summer - don’t remember the winter dynamic, we were talking about summer crops) just to the east. I’d always figured you’d have to go a long ways further inland, like even Death Valley seems too close to the coast to be that hot. But it is. So, Southern California, within sight of the ocean, is probably an extremely nice place to be. Northern California also pretty nice, but kinda cool (though every summer here in North Texas when we’re having lows of 87 and highs of 105 I swear I’m going to move to San Francisco - I don’t really mind putting on a sweater in the evening…;))
Mind you, I’ve never been to California, this is all hearsay. And I have been to Hawaii (3 times all at different times of the year) and the first thing I noticed is that virtually every location that would be enclosed and air-conditioned in Texas (and enclosed and heated in most of Europe and lots of the continental US) was open-air, including every mall and the Honolulu airport. It was fantastic.
eta: wrote this before mlees’ post
You are mostly correct, you need to be within a few miles of the coast to enjoy the moderating effects of the marine layer.
I work right on the beach, but I live about 20 miles inland, were the summer temps get into the high 90’s easy. The temperature can be 10 degrees different within that distance.
(All temps on Fahrenheit scale, as I am metricly impaired.)