For long stretches, I know Miami - Atlanta and Cincinnati - Atlanta have already been mentioned, but I LOVED the drive between Orlando and Chattanooga when I used to make it when I lived in Orlando. I miss it.

Mostly I miss living in Florida I will admit, lol.
I made that drive to visit my mother, late sister, and younger brother before Allegiant Air started flying that route every six weeks.
South of Atlanta: Even though the route was VERY heavily travelled, you could dependably count on finding a convoy driving 85 - 90 mph. Trucks were required to stay in the right two lanes on I-75 and I-475, and the Florida Turnpike was (slightly) less congested than I-75 despite being only 4 lanes (not much). And I was a Sunpass customer, so I did not have to wait at the Leesburg plaza, I was able to sail right through (albeit at 30 mph, but still better than stopping and waiting) and my exit was Sunpass-only.
Atlanta itself: FUN! I had a small car, so if I was travelling solo, I was able to switch lanes easily. If my late sister and / or younger brother decided to fly one way to visit me to visit theme parks and fly the other way, I could use the HOV lanes one way.
North metro Atlanta (namely I-575 - Cartersville): The ONLY part I did NOT like. Not enough lanes for the congestion, moved very slow usually.
Cartersville - Chattanooga: Alright, I didn't like it as well as I did south of Atlanta (or Atlanta itself), but certainly better than North metro Atlanta (especially I-575 - Cartersville!)
For scenic stretches, few can beat I-840 between I-40 west and I-65 (the westernmost arc). Goes over the Natchez Trace Parkway. And it's lightly travelled, especially for going through a metropolitan area of 2.1 million.