Prior to the Elite 4, your options are quite limited. Only (I'll just be naming the fully evolved Pokemon here) Beartic, Lapras, and Vanilluxe can be obtained if the season is not Winter. Seeing as Cubchoo can only be found in Hidden Hollows and Lapras from Bubbling Water, Vanilluxe might be your best bet. If you have the good luck of finding a Cubchoo in a Hidden Hollows, or if it is Winter, I would use that over Vanilluxe as its movepool is better for ingame use. But use Lapras if you have the luck to find one, as it is an excellent Pokemon for ingame, with HMs and great type coverage.
So Vanilluxe if the season is not Winter and you can't find a Lapras or a Cubchoo.
Beartic over Vanilluxe if you can find a Cubchoo either through Hidden Hollows or through it being able to be found in the Wild in Winter.
Lapras over Vanilluxe or Beartic if you have the luck of finding one.