The best route from I5 is highway 126 from Eugene to highway 20 that leads to Bend, then highway 97 to Klamath Falls. You will still be driving over the Cascade Mountains, but there is no way to avoid them. Other routes such as highway 20 all the way from Albany are more curvy and slower. Highway 22 from Salem would be my second choice if approaching from the north. If starting from Portland, then highway 26 to highway 97 in Madras is the best route.