Here's a brief geography lesson:
Closest to you are the Appalachians, with the highest, twistiest roads around Great Smokey NP. So by going NW via Alabama will be flatter than via the Carolinas and Kentucky.
There are some hilly areas in northern Arkansas and southern Missouri. On the other hand, most of Illinois, Iowa, Kansas and the Dakotas are as flat as Florida.
There are some minor climbs in/out of major river valleys - Mississippi and Missouri (in the I80/90/94 area).
The Black Hills are first mountains you'll see on I90 - which skirts those hills. But seeing the tourist attractions (e.g. big rock faces) requires driving into the mountain valleys - along with all the other tourists.
The Big Horn Mtns cross the NC part of Wyoming, and the first western mountain range that most eastern tourists see. I90 skirts around the north. US16 is the easiest of 3 routes up and over these mountains.
Yellowstone is a mountain top - so all routes involve some sort of climb. US16 from Cody has a long steady grade, but no worse than the Big Horn piece. Most of the other routes into Yellowstone have more gradual grades (except the NE corner).
The drive from Yellowstone to Glacier follows the continental divide, on one side or other. Going to the Sun in Glacier is the highest, most spectacular divide crossing north of Colorado. It is also the most restrictive. US2 around the south side of Glacier is one of the easiest highway divide crossings.
I90 has several passes in Montana, with the I15 intersection in Butte, a mining town. Mines are located where the ore is, even it is a mountain top.
US89 crosses the small Little Belt Mtns south of Great Falls. It also winds through foothills between Browning and St Marys. So many RVs prefer to skirt this section.
