Can't say I'm super well versed in metro systems across the globe, but I loved the train system in Japan, including the Tokyo metro.

The trains go everywhere. They're frequent, on time, and clean. And!! Suica / Pasmo / whatever cards. Amazingness. They work on the subway. They work on above-ground trains. They work all over the country in different metro areas. They work at vending machines. They work at convenience stores. Even some restaurants in the train stations. Don't have to worry about carrying around change or credit card acceptance or anything like that. Super practical.