Hawaii: Like BRGT350 also said, the Road to Hana around the island of Maui. It also sticks to my memory because I just had my permit at the time and my dad let me drive most of it in the rental car.
Southeastern US: North GA, TN, and NC roads such as Suches loop, TOTD, Cherohola skyway, etc.
West US: In summer 2017, I took a trip through Phoenix, Kanab, Flagstaff, Laughlin NV, Zion, around Yosemite, and Mammoth Lakes before finishing in San Francisco. Many amazing roads out there but the two that stick out to me the most was the path through forest and mountains after passing the Sierra Nevadas while on the way to and after Mammoth Lakes, and one really curvy, desert mountain road at high altitude as we came into California. It definitely wasn't the fastest point A-B route. Forget the name, but I think it was Westgate pass or something? IIRC, in August it was over 100 degrees F, ~8000 feet elevation, and we were basically in the middle of nowhere. Loved that trip.
Greater LA: Went to a wedding last year in Huntington beach and stayed for two days afterwards to explore in a Turo ND Miata. It's a long drive, but I really enjoyed Angles Crest, as well as Topanga Canyon, and the drive up Las Flores Canyon. Lots of other roads I can't remember right now as well. You guys are really spoiled
Germany: German backroads. Again, I can't name specific ones but the hilly roads connecting both Koblenz and Saarbrucken to the Nurburgring and other small villages were great. No traffic, super smooth, great sight-lines. Ditto for the drive from Munich to Zugspitze Peak. For that trip 3 friends and I rented a 750d (i6 with 4 turbos) to take all over Germany. That car was amazing.