I prefer GiftCardZen because they usually have good rates, electronic cards come quickly and I like their guarantee. I've had to claim against it before because a gift card balance disappeared, and they processed my refund quickly and without hassle.
Whatever site you use, check the guarantee CAREFULLY and read the full agreement, not just any summary-type info they have on Help pages and the like. Make sure they cover the gift card type you are purchasing, and know the timeline they will cover it for. (Cardpool I know guarantees only physical gift cards and not electronic gift cards, but you wouldn't know that unless you read their full terms & conditions - look for that sort of thing).
Verify the gift card balance as soon as you get them, and spend them as quick as you can.