Whilst in Montreal, I rode the Metro to get to a big market. It got me wondering how many subways/light rails/public transit systems I've ridden:
DC Metro
Boston T
NYC Subway
BART and streetcars
NOLA streetcars
Chicago El
Paris Metro
London Metro
Montreal Metro
Just under ten. How about you? Ridden a lot of subways? Like 'em? Hate 'em?
My own EU country plus all its neighbours, so make that 5-6 just there (all modes from city buses, long distance buses to S‑bahn, trams, trolleybuses, metro, rail, ferries, cable cars)
Paris (all modes, including the city's excellent Vélo bikes)
London (all modes, Eurotunnel to get there)
Ukraine (all modes including marshrutkas* and ferries)
Romania (all modes)
Hungary (rail plus city transport)
Moldova (buses, trams, trolleybuses and marshrutkas)
Transnistria (marshrutkas)
Bosnia and Herzegovina (trams, buses, rail)
Albania (buses, ferry)
Croatia
Serbia
Greece
Italy
Spain
Georgia (the country, not the US state) (metro, buses, marshrutkas, rail)
Egypt (metro, buses)
Iran (metro, buses, rail, tuk‑tuks)
China (all modes except rail I think)
…need I go on? I don't think I can even count that high!
As for a favourite, I'll have to think about it more, but the most
memorable would probably be marshrutkas in Georgia – their drivers are absolutely crazy on the crazy mountain roads, overtaking into a turn they can't see into while relying on a horn to clear the way – and one old passenger ferry in Albania, as it's cabin was just a body of an old German bus welded onto a flat ferry, complete with the windshield and seating.
* a private minibus that usually waits until it's full before departure and stops anywhere along it's route when flagged down, with a somewhat loose schedule