Solitude, by a long shot. Beautiful natural scenery, an orderly well kept city with plenty of amenities, a relatively safe surrounding area, reasonably calm weather patterns, stable government. Probably the highest standard of life for a regular peasant, provided you're not a Stormcloak revolutionary.
Windhelm and Winterhold have violent inclement weather, and besides that, Winterhold is a burned out wreck for non-mages, and Windhelm is poorly maintained, poorly governed, and has a sizeable slum. Falkreath is poorly protected, is in the heart of dangerous wilderness, and has an undesirable element close by (Dark Brotherhood). Markarth has questionable government, is built overtop of lethal ruins, and suffers constant Forsworn invasions. Riften is a hotbed of crime and corruption. Whiterun is probably the only other significant holdfast that could give Solitude a run for its money. But it has that faction war inside the city walls, and on at least one occasion we see a giant blundering into the farmsteads, so it loses by a nose.