I-80 is the fast way and flattest way.  That said, I'd much rather take the
scenic route and go Hwy 50 to I-70.  Great Basin NP (a real sleeper in the
NP system) and  Arches NP and many other parks around Moab on this route. 
I-70 through Colorado is much, much more scenic than I-80 up north.   Plus,
there's nothing like a drive on Hwy 50 through Nevada to make one realize
that there really are some serious wide open spaces left in this country.  
They don't call it the "lonliest highway" for nothing.

If you do take I-80, cut south in eastern Utah to Dinosaur National
Monument (another sleeper) and follow US 40 through Steamboat Springs into
Denver, detouring through Rocky Mountain NP if you want.  

Enjoy the ride.

Bill
www.geocities.com/yosemite/gorge/1706