The prettiest is 101 down to Yachats. If your in a hurry this is not the road for you.. It goes through a lot of towns very pleasant drive.
The shortest is I5 to Corvallis over 20 to Yachats. Thats not very pretty and the road is slow but very nice.
In my opinion I5 to 126 to Yachats is the hardest because you will be going past Florance and just past Florance there is the Sea Lion Caves and Hecta Head light house. Very very tight road. All roads are doable but there is a very tight spot at Hecta Head.. Hope this helps.