Any coach fare, as far as I know. From the wiki in the Companion Fare FAQ:

What class of fares can be booked?
All fares can be booked from "hot fares" to "first class" but only on Alaska/Horizon operated flights. It is not eligible for AA, DL etc operated flights. (coach fare only with new certificates issued after Aug. 1, 2012). You cannot purchase F but you can upgrade to F after purchase or try your luck at free upgrades if you have status.

https://www.flyertalk.com/forum/alas...plan-visa.html

Edit: I had never tried it or paid attention to it, but even Saver fares work.