The RATP of Paris is my favourite of which I've experienced so far, it covers the entire city, has reasonable fare, and is pretty easy to understand.
However, back home in Canada, I am still loving the TTC of Toronto. Last subway departs at 1:30, with bus routes running afterward, meaning it is completely possible to live a normal lifestyle without owning an automobile, something that is VERY difficult to do in North America. Its fare of $2.75 can be a bit pricey, once the fare is paid, the TTC rider can remain on and transfer between routes for as long as they desire to do so. The TTC is also undergoing a massive renovation and upgrade project, with plans to add to the subway system, as well as add LRT routes that service the areas of the city without subway stops, as well as eventually extending LRT/RT routes into the neighbouring cities of Mississauga and Vaughan/Richmond Hill.
Toronto also benefits from the GO Transit System, which is a regional network of rail and bus transport. All of the Greater Toronto Area is covered, along with neighbouring cities such as Guelph, Hamilton, and St. Catharines/Niagara. For a very reasonable price, you can commute to downtown Toronto without the hassle of driving on Southern Ontario's severely congested 400 series highways.