I sort of collect top up cards for metro/mass transit systems in cities I have been to. I always try the metro in cities I go to that have one.
My favorites:
- Tokyo: absolutely insane in the level of precision, user friendliness, timing and coverage of the city. I could ride it for days
- Seoul: covers so much of the city, you are always near a station and there is always a station where you want to go
- Paris: same as Seoul
- Hong Kong: very practical, works wells, clean, efficient
- Singapore: same
- Lisbon: I like the line name scheming and logo, find it somewhat relaxing

The worst:
- Shanghai: very disappointing for an Asia subway: not clean, not very user friendly, rude locals
- Xi'an: same as Shanghai
- Lyon: very uncomfortable seats