Out of my own experiences: BS, IG, GG and GMG are good.

The rest range from dodgy to very dodgy.

I've had a good time buying from Indie Gala, I'd really recommend it. Now not that I'd tell you to avoid, but I was parctically robbed by Nuuvem, they gave me a used key and refused to refund.

I think all the ones I use or have used in the past were already mentioned.
Humble Bundle / BundleStars / GamesPlanet / IndieGala / IndieGameStand / GamersGate/ GreenManGaming...
Maybe you should split the list between official stores and unofficial ones. Also, you didn't put Steam xD

now when g2g don't have anymore paypal and credit card methods of payment all the rest they have is unsafe (skrill, alipay - chinese) or unprofitable (sms, bitcoin, paysafecard). Here g2a against skrill: https://www.reddit.com/r/g2a/comments/47xqg3/i_cant_buy_anything_using_my_skrill/ - 4 months ago.

I've used Humble Bundle a lot of times, Bundle Stars for some bundles and Indiegala for one bundle, i've used Green Man Gaming too, when they gave 1$ free buying Bioshock Infinite, i think those sites are reliable.
I've used G2A too, most of the times the key was good, when not, i opened a "Dispute" with the seller, and i've received a refund or a new key, one time the seller didn't cooperate, after 8 days G2A changed the "Dispute" to a "Case", and they give me a refund, i never used the Shield option, but it's true that i've never bought expensive games on G2A.
Never used other sites, so i cannot recommend others, sorry.

g2a mention detected:

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