Wasnt surprised about Vienna's omission from the responses (with only a few passing mentions). It's the best but also most underrated system in the world:
1. Extremely good value. Basically 1$/day for annual pass covering all modes of transport in main zone (which is basically all the residents)
2. Frictionless use - no barriers or turnstiles anywhere. A huge consideration IMO
3. Very high general standards (modern, clean, etc) for the stations and the equipment. With a few (clean!!) historic trams mixed in
4. Some station with good historic and artistic value
5. High punctuality, coverage, and capacity - rarely a sardine situation, and definitely nothing like the London/NY/Paris rushhour situation
6. Virtually no beggars or buskers, despite
#2
7. Also extremely safe - no risk of theft like the Barcelona of the world
8. Easy access to airport. Additional $2 on top of standard fare
I could go on and on but for me it's a runaway victor