Ranking was one of the many text commands in Transformice. It displayed the current highest scorers in the categories of mice saved, most cheese collected, and most cheese collected first. Ranking was accessed by typing /ranking in chat. Currently, the ranking is down for all servers, because of the lag created that disconnected players.
There is currently an unofficial leaderboards at fansites with use of the Transformice API, such as cheeseformice.