I-90, I-10, I-70, and Route 66 are some of the most well known east to west routes. Personally, I like I-90 and I-10 the best.
Some of the best spots along I-90 are Badlands National Park, Boston, Chicago, and Bighorn National Forest. In Wyoming, I-90 ends, but the smaller highways will take you to Yellowstone and Grand Teton National Park.
I-10 has some great scenery, and doesn't have as much directly on the highway, but puts you nearby some of the country's greatest things to see. Grand Canyon, Carlsbad Caverns, and Big Bend National Park are all within a day's drive of the road. There are also some big cities like San Antonio, Houston, New Orleans, and Palm Springs right on I-10.
If I were to choose one or the other, I'd pick I-90 in the warmer months, I-10 in the cooler months.