I-99 from State College to Altoona is only rivaled for scenery by I-70 from Breezewood to the Mason Dixon Line. Even then, it is not much of a contest as I-99 has mostly unobstructed views of the valley below unlike I-70. Some of my favorite drives have been southbound on I-99 on a clear day when the morning fog has yet to subside from the valley below. On those days, it genuinely looks like you are on a floating landmass in the sky.

Since others are also including non-Interstate divided highways in this thread, I will also mention US-219 from Ebensburg to Meyersdale, which has its fair share of stunning views as well. It is a bit under the radar given that it is not the most frequently traveled of roads in the state, and, to be honest, I have no idea why it is a four lane divided highway to begin with. But, it is a very pretty drive nonetheless, especially in the fall, and I love driving that stretch on my way to the Youghiogheny River watershed for my leisurely sojourns with the river.