I've been looking into this for a couple of years. I factored in COL, overall tax situation, climate (high humidity is out), culture, beauty of the natural surroundings, distance to major metropolitan area(s) and distance to my closest family. Your criteria will be different, of course. Here's my short list:
1. Carson City, NV: Not all that far from where I live in southern Cal. Very close to beautiful Lake Tahoe and the eastern Sierras, but on the Nevada side, so a much better COL and tax situation. Decent climate, some snow in winter but not bad at all. Even in the Summer when it's hot during the day it cools off at night, and it's a dry climate. No big cities nearby, though. The closest decent size city is Reno.
2. Flagstaff, AZ: Relatively low cost of living, especially compared to California. Tax situation is decent. Lots of great natural beauty nearby (Sedona, Grand Canyon, Lake Powell, the southern Utah national parks). Small town feel, might be culturally lacking but Phoenix is about two hours away. Weather is pretty decent. Again, hot in Summer but it cools off nicely in the evening, and it's dry. A day's drive from soCal.
3. Longmont/Loveland/Ft. Collins CO: COL is pretty decent, as is the tax situation. Off the hook beauty all around. Close to Boulder, and not too far from Denver. Snow in winter but again, not arctic-like conditions. Lots of parks, biking, outdoorsy stuff to do. A great choice, but it's far from family.
4. Cheyenne, WY: The economy is doing great, great tax situation, it's listed as one of the "best run" states in the US. Lots of natural beauty around. Fort Collins, CO is less than 50 miles away. Edging further north now, the average low in December is 17.3 degress F. I don't know much else about it, but looking at it on the map it seems pretty isolated. Not a lot going on to the east, west or north.
5. Austin TX: As others have stated, an awesome, happening, city. Texas is the big deal right now on all of these top 10 cities lists. The economy is booming, the tax situation is very favorable. Great music scene. I prefer access to alpine-like forest areas, and none of those are close, although Austin is in Texas' "hill country". The deal breaker is the heat. I just couldn't live in a hot, humid place, I would be miserable.
One thing about these Best Cities lists that come out every year is that they tend to be very trendy. You look at one from three years ago and it's amazing how far a city can drop, but I'm sure the living conditions have remained roughly the same. Texas, Wyoming, Utah, the Dakotas are making top 10 lists right now, but will they even make these lists in five years? Plus, the people making these lists have no investment in them, no skin in the game, so to speak, so what do they care if their criteria are dumb or irrelevant? My point is, they might be ok to look at to get ideas, but you can't take their data seriously.
Here are some actual numbers based lists--the 50 states rated by tax burden and by how well the state is run (GDP growth, budget shortfalls, unemployment rate):
http://best-state-taxes.247wallst.com/http://best-run-states.247wallst.com/