Custom Race mode, livery editor, Scapes, watching Dynamic Scapes, Museum, and listening to the music are my favorite features.
Custom Race mode. It's what I have been wishing to appear since the game was revealed. Very reminiscent of GTPSP's race mode except with additional things added like fuel consumption, tire wear, the ability to choose whether it's a timed race or a lap race, and the AI being decent compared to GT League's AI are enough to say its my favorite feature.
Livery editor. This feature I'll have to say its the best livery editor I've ever used in any racing game that I have played with a livery editor. Add in the decal uploader which makes it better for me to make liveries. Last time I experienced a livery editor that I enjoyed so much would be Midnight Club Los Angeles' livery editor.
Scapes. This I love it because it allowed me to take better pictures and such without having to do post game editing. Like what some peeps said, it's the Lightroom equivalent if not better for some.
Watching Dynamic Scapes. I can just treat the game as a screensaver for like 20 minutes or more and I can just relax.
Museum. Just like GT5 it made me learn some stuff.
And listening to music. Ever since I discovered Transmission Mode music, it just made it a lot better. Kudos to PD for introducing me to Hotel Costes music.
Honorable mentions go to Sport Mode Daily Race and FIA races because I'm only a part timer in these events. Oh and Brand Central as well because of how much it reminds me of GT4's dealership brand screen but better.