Metro systems that have stood out in my experience:

Moscow- I could explore that system for days, marveling at the Socialist Realism and uniqueness of some of the older stations. Quite efficient system, too, though not terribly welcoming for non-Cyrillic readers. I picked up basic Cyrillic specifically for this metro system...ok, and for menus.

Pyongyang- stations are named after ideals, and also have beautiful artwork.

Mexico City- at every other station, it seems a new vendor/busker boards...not that I like it, but watching people's reactions can be entertaining. Practical icons representing individual stations, too (also in Guadalajara).

Hong Kong- the most overrated system. Rude locals, unnecessary tvs, always a logjam at the narrow gates, can't add just any amount to Octopus cards (at least with cash), ridiculous eating/drinking policy, etc.