Underground/Metro systems you have travelled on

[Eleanor Martins]

Munich, Singapore, Bangkok, and Portland's streetcars, because I'm running out.

[Spaghetti Cat]

Just DC/Virginia

[Hash]

The so-called Mitterrand line in Paris is amazing, I like it.

Rennes metro is excellent too, it's all automatic, so there is no risk that the lazy communists will go on strike.

[Hatman 🍁]

Toronto and Montreal.

[Platypus]

In terms of non-bus public transport:

Melbourne (trams and trains, including underground stations)
Sydney (ferries and trains, inc underground)
Santiago (bus-type trains...fast, clean and on-time, easily best system i've come across, largely underground)
Buenos Aires (subway)

[TeePee4Prez]

Philadelphia
New York
Boston
Washington, D.C.
Atlanta

[12th Doctor]

New York, Pittsburgh (pretty rare, since most people who have been here all their lives haven't), Washington DC, Paris, Amsterdam, London.

[Brittain33]

Lille
Strasbourg (not underground, but cooler than normal trams)
Paris
Brussels
Antwerp
Amsterdam
Cologne
Frankfurt
Berlin
Copenhagen
Barcelona
London
Manchester (see Strasbourg)

Boston
New York
Washington
Philadelphia
San Francisco

Montreal
Toronto
Mexico City

Trams/weird systems in Laon, Gothenburg, Mulhouse, Ghent, The Hague, and probably other French cities but it was 11 years ago and I don't remember what was a bus and what was a glorified bus.