The best line is the one that goes from Paris to a destination you want to visit. You are not going to notice any small difference in top speed.
Trains from Paris to Geneva will run at the speed of ordinary trains for part of the journey, as there are sections of track that are not suitable for high speed. The longest continuous high speed run is Paris to Marseille or Montpellier.
The line between Paris and Maçon is, for the most part, the oldest of France's high-speed lines and isn't, therefore, the fastest. In particular, trains have to slow slightly as they crest rises in order to ensure that they don't take off.