Obviously, the answer to this question about "most interesting" is gonna heavily depend on your specific interests, but if you're interested in 3D graphics (and creating them yourself), Blender 3D is excellent (and Free[dom]; and free[of cost]), and https://www.youtube.com/user/AndrewPPrice (Blender Guru) has excellent tutorials to help you in your journey from "noob" to "guru". ;~)