Best route to Oregon Coast from I-5

From and to where? My preference for getting to the central coast has been 22 out of Salem. Far less traffic, nice drive with only a couple of minor hills.

The "best route" is Astoria and south. It's the "best" because most of the turnouts are on the ocean side.

US 20 from Corvallis has a new section that shortcuts most of the really curvy areas and is 2 lanes over the steeper parts. In fact I think all of 20 from Corvallis to Newport has 2 lanes on all the uphill portions.

OR 22 to OR 18 is the lowest pass at about 800 feet. Good road. If you choose to stay on OR 22 into Hebo it is quite twisty. OR 126 is a good road with the summit at 1000 fee, IIRC. Little bit curvy but not bad. OR 138 into Reedsport is a good road and lot people prefer it, myself I don't but nothing wrong with it. In reality it is about a toss up on what one you take. My only advise of what not to take is OR 36 from Juction City to Mapleton. Doable but it is a goat trail. Not a bad logging road though.

Route to Oregon coast-
My choice is always hwy. 138 to hwy 38 to the coast. Turn west at Sutherlin. RV friendly, good road and pretty scenery.

Agree with Mark Twain. That's the way we go from I5 to the coast.

I guess it depends on where you are going from and to. I like 22 from Salem to Lincoln City. The maximum elevation on that route is 600 ft. The maximum elevation on route 20 is 900 ft., and on 26 it is 1,600 ft. If you have trouble climbing hills, elevation is your enemy. I don't know the maximum elevation on route 138/38. You might want to check that.