I'm hardpressed to think of one for Oregon. Best I'm coming up with is WA 14 (with possibly WA 4) or US 12.
You could make a case for I-15 bringing traffic up from Salt Lake, Denver, Las Vegas and Phoenix.
That, or possibly I-90.
I could accept those.
I could also make the argument for I-80. Long distance traffic (eg. anywhere east of Utah) traveling to Oregon will likely use I-80 (more likely than I-90). I-80 provides connections to Eastern Oregon via US-95 and US-395, though admittedly insignificant compared to I-84 (most traffic using those two routes will be crossing I-80 more than actually using it, eg. traffic from Reno or Vegas). And adding in this one puts it over the top IMO, it's part of the route between the San Francisco area and anywhere in Oregon, along with I-505 and I-5.
I think it's more important than I-15 at least, but I-90 and WA-14 are just as good of contenders. WA-14 gets points for being a major detour route if I-84 has to close in the gorge. Whether it be for general winter weather, wildfires, derailed trains, etc., I've seen WA-14 used as a detour for I-84 many times.