Dota 2 is the first game that recieved the Source 2 Engine treatment. Source 2 includes technology for rendering improvements, performance optimizations, higher fidelity content, and richer, more dynamic games.
Steam VR Home an evolution of Valves Destinations toolkit also uses Source 2 to bring users to a multitude of locations both official and user created.
Half-Life Alyx is the first game to showcase the engines true abilities through a first person viewpoint and utilizing more true to life colors (when compared to dota2) and physically based shading. The game also demonstrates the work Valve has put into creating rich and vibrant VR experiences that the market needed in order to continue to grow.