To me memorable could be either pleasant or unpleasant, or some unique feature which made it memorable.
Tokyo- memorable due to its sheer size and volume of passengers transported
Moscow- memorable for some incredibly beautiful stations
Mexico City- likewise for efficent and some very attractive stations
Montreal - for its rubber wheels (ie relatively quiet ride)
Buenos Aires- although not extensive, it is one of the first in the world.
London/Paris- for their size and age and efficiency
New York- just because it is New York and the interesting cast of characters that ride the subway.