The French Defense
The main variations of the French Defense are the Paulsen, Tarrasch, Advance, and Exchange Variations. The French Defense allows white to take control of the center early and then tries to break it down with pawn pushes like d5, and eventually c5. The push to c5 is a common theme in the French.
Paulsen: This is the most popular and arguably the most challenging for black to play against.
Tarrasch: The second most popular option for white.