Looking at climate through the lens of degree days (base 65 F US, 64 F/18 C INTL)
Cooling degree days occur any time the mean temperature of the day is more than 65 F/18C.
Heating degree days occur any time the mean temperature of the day is less than 65 F/18C.
A good way to measure climates is to look at the mean HDDs and CDDs during a typical year. We can also sum the two to get an idea of how relatively comfortable a climate is, and how much energy is required in that climate to heat and cool a building through a typical year.
Hottest locations in the world by CDD:
Mecca, Saudi Arabia 8,176 (hands-down the #1 hottest large city in the world)
Khartoum, Sudan 7,833 (hottest national capital)
Bangkok, Thailand 6,680
Singapore 6,192
Manila, Philippines 6,027
Jakarta, Indonesia 5,905
Manaus, Brazil 5,861
Mumbai, India 5,756
Recife, Brazil 5,718
Dubai, UAE 5,702
Death Valley, CA, USA 5,600 (hottest location in USA, uninhabited)
Lagos, Nigeria 5,577
San Juan, PR, USA 5,588 (hottest large city in USA)
Key West, FL, USA 4,825 (hottest in mainland USA)
Phoenix, AZ, USA 4,620 (hottest US city with metro >1 million pop)
Coldest locations in the world by HDD, isolated outposts like Eureka, Canada not included:
Verkhoyansk, Russia 21,322 (coldest city with > 1,000 pop in world)
Barrow, AK, Alaska, USA 19,428 (coldest in USA)
Noril'sk, Russia 18,307 (coldest city with > 100,000 pop)
Yakutsk, Russia 17,788
Iqaluit, Canada 18,211 (coldest over 1,000 pop in Canada)
Churchill, MB, Canada 16,322
Yellowknife, NT, Canada 14,861
Fairbanks, AK, USA 13,531
Ulaanbaatar, Mongolia 12,537
Leadville, CO, USA 10,877 (coldest town in lower 48 USA)
Novosibirsk, Russia 10,650 (coldest metro > 1 million pop)
Winnipeg, MB, Canada 10,400
Edmonton, AB, Canada 10,274
International Falls, MN, USA 10,080
Moscow, Russia 9,168
Minneapolis, MN, USA 7,574 (coldest metro > 1 million pop in USA)
Least amount of HDD, CDD, Total
Uruapan, Mexico 276, 429, 755 (most "agreeable" climate in world in terms of temperature?)
Huambo, Angola 429, 364, 793
Guatemala City, Guatemala 62, 749, 811
Nairobi, Kenya 155, 742, 897 (most "agreeable" major city in world?)
Mexico City, Mexico 954, 122, 1,076
Cochabamba, Bolivia 934, 153, 1,087
Addis Ababa, Ethiopia 1189, 0, 1,189
Iquique, Chile 766, 677, 1,443
San Diego, CA, USA 1166, 802, 1,968 (most "agreeable" major city in US?)
Degree days are all based on degrees Fahrenheit