Here is the largest, biggest, most famous, most crowded line in Tokyo, it is technically NOT a metro line as it is operated by JR and you will need to commute inside JR stations to catch it, but it allows to commute around town
This is the circle line of Tokyo, connecting 29 stations in exactly 60mn for one loop. Mornings and Evenings are to be avoided if you can as can packed like hell!!
but great way to explore Tokyo, I have made a to-do list of myself to stop at everyone and walk around