The RX 580 and GTX 1060 6Gb are in that price range and trade wins depending on the games. Nvidia's new line of cards is out, but they are scarce and expensive, and only the top end cards are released.
If you want the RX-580 (8GB) for $300, I would look at the ones with the
highest core clock out-of-the-box
for that price. They usually have the highest binned GPUs, meaning you may get better OCs if you are into that. Those would appear to be Asrock cards.
Since Asrock entered the gfx card field, I'm kinda interested in their product. If it is as good as their motherboards are, it might be the best choice.