Technically, I’d say that GOG does, as you can just download and back up all the installers for the games. Wouldn’t even matter if the company went bankrupt or even if the entire internet died completely. You could still install and play the games just fine.
When you buy something digital, since it’s expensive, you want an assurance that the platform would honor your access for many years.
Valve has the best chance of that.
Technically, I’d say that GOG does, as you can just download and back up all the installers for the games. Wouldn’t even matter if the company went bankrupt or even if the entire internet died completely. You could still install and play the games just fine.