Which countries are most climatically favorable - that is, many sunny days and a stable daytime temperature of 20-25 degrees?

Auspiciousness is a very subjective criterion. In principle, according to the given conditions, everything that exists has already been named before me.

But I don't really think such conditions (for example, Malaysia, Singapore especially) are very favorable.

In general, both me and the author of the question should suit San Francisco. There is a very specific microclimate because of the mountains around and the bay. In summer it is rarely higher than +25, and in winter it is rarely below +10, the climate is not dry, not humid.

There are even colder places, but also with a pleasant mild climate - for example, the Faroe Islands or Great Britain. But this is already the case, in addition.

The flattest temperature on the islands of Oceania.

For example, on the Mariana Islands, daytime temperatures at any time of the year are rare when it is other than 27 ± 3 ° C

Provence! It is sunny and very warm there more than 300 days a year. True, in winter it gets colder up to +15 degrees. But, in any case, after Russia, I was delighted with their climate.

Stability of temperature and a clear sky in general contradict each other: cloudiness makes it difficult for the penetration of solar radiation and heat exchange with the "upper" atmosphere, working as a natural greenhouse. At the same time, low humidity lowers the perceived temperature, so in general dry places are better for living. I think California and the Mediterranean climate in general (eg Adelaide, Perth) are best suited to your needs. Perhaps the southeast of Australia (Sydney, Melbourne), the southeast of Brazil (Rio, Sao Paulo) are of interest, although it is more rainy and humid there. You can consider Southeast Asia, especially those that are closer to the equator (Singapore, Manila, Jakarta), but in this case, prepare for high humidity, tropical showers and typhoons.