In my current game I've had great use from Kabul, Buenos Aires and Zanzibar. I'm playing a domination victory, and with the help of Kabul I now have units with seven promotion levels (industrial era). A 7-promotion Crossbowman and a 7-promotion Musketman together can just stroll in and take a city in maybe two or three turns by themselves. And Buenos Aires and Zanzibar... Well, if you conquer like three cities per turn (which was the case for a while), you need a lot of amenities to keep everyone pleased with your population increasing so rapidly. I'm also spamming Entertainment Districts by the way. And I unlocked Zoos this turn. Oh, and when I got into a war with Geneva's ally I didn't even think about not capturing them, because I couldn't have peace if I tried with my -100 warmonger penalties with every AI.