I lived in Moscow for a while as a kid and the Metro system was such a joy in retrospect that it's really soured me on some other cities' transit systems. Living in Seattle right now while we struggle to achieve a single-route light rail system is especially saddening.

Crazy that Moscow's system is almost 100 years old, dating from 1935. (I just checked and London's Underground has it beat by a margin though - 1863)