I was just wondering this. It seems a little confusing. I thought it was based on how many people marked a review as helpful but that doesn't seem to be the case. In some places the "most helpful" review seems to be one that simply contains notes like "I used to play this game and I bought it and love it!" over other reviews that contain, y'know, an actual review of the game.

I think it's determined by their KDR in CS:1.6

Its a combination of how many people have marked it as helpful as well as the number of people who marked it as not helpful. I don't know the actual algorithm used, but what it basically means is a review with 20 helpful votes can actually end up being marked as less helpful than one that only has 5 helpful votes.

The algorithm is unknown, but it is however known that people disagreeing with the review weights more compared to the amount that agrees.

What's also strange about the most helpful system is how the voting works... I've noticed that you can vote on as many browsers as you want without being logged in. Personally, I think you shouldn't be able to vote on a good review unless you're logged in... that'll eliminate excessive voting.

Wow, that's surprising. Yeah, I'd agree they need to fix that.
It would be great to have someone explain how it works. It's just odd seeing some reviews labeled as "most helpful" when they're one or two sentences about how someone used to own the game and they really really like it. I suppose to some extent "helpful" is a matter of opinion, but generally it would make more sense to have the more detailed writings get labeled that way.

It definitely would make more sense to have a helpful review be one that was detailed, but it would be difficult to measure that quantitatively. I mean, there would be no smooth/easy way to label reviews as helpful or non-helpful based on sheer number of words. Selecting key words as helpful would introduce unwanted bias from the creators of GOG.
There's really no easy way around this. I guess they could make the name of such reviews different; instead of "most helpful", they could be called "most popular", since that's truly what they are. Helpful is fairly subjective, and judging something like that based on vote proportions seems a little odd.