Singapore's subway is one of my favorites. The antique line in Buenos Aires (Linea 1 IIRC) is interesting to ride.
In the USA I actually like Chicago's system of busses and subways. Yeah, the subways are garbage pits and the elevators are full of urine (Chicago Street station, Red Line, 2 days ago), but the system gets me where I want to go without having to rent a car.