1:12 so you can shoot bullets from 320 gr and lighter. 1:14 would theoretically provide slightly lower pressure and slightly higher MV, but in practice it would fall within the normal variation in MV that depends on your barrel, powders, primers, etc., but would potentially limit you somewhat with the heavies.
For those who are concenred that bullets will be over stabilized and/or are concerned that the bullet will come apart due to excessive spin, the latter has been produced in smaller calibers with very fast twists at very high MV's, but that won't ever happen in a 35 Whelen.