The Line 13 is the most notorious for being packed in mornings and evenings, with the section between Porte de Clichy and Saint-Denis being the busiest.
Line 4, which runs north-south right through the middle of Paris, passing by several main train stations and transportation hubs on its way, is often very crowded as well.
Line 11 has become increasingly crowded in recent years as more and more move into the 19th and 20th arrondissements (some of the most affordable neighborhoods in the city).