A Brooks saddle. It’s nice to be able to ride all day in comfort without needing cycling shorts.

The 'Biggest difference in your riding experience' can also include a pair or two of quality shorts, and if you ride in inclement weather, other quality cold weather clothing.

A couple of years ago I bought a Canyon Grizl gravel bike. I looked through all the various versions and the one I bought looked perfect. However, after I started riding it I found the saddle to be very uncomfortable even though I thought it would be very good. I replaced it with an $18 saddle I had bought from Nashbar 20 years ago. Much better

Biggest difference in riding experience?

absolutely without a second thought.....a rear hub motor.
Then, tires/touchpoints/wheels/weight shaving

touchpoints would include the software as well as the hardware. Expensive shorts, shoes, gloves to match the perfect saddle, pedals, handlebars.

and the latest helmet, because everyone would stare. That would be a big change over being ignored, as are most cyclists on the road.
https://cimg3.ibsrv.net/gimg/bikefor...20934ed5c6.jpg