I think it's pretty safe to say that reference cards will overclock the best. Reference cards are USUALLY (not sure if MOST of the time) have a AMD badge on the PCB bottom of the card near the part of the card where you stick it into the pci-e slot on your motherboard.
This is because these companies make revisions to the graphics cards and often skimp on quality parts, opting for lower prices for you to purchase at a discount. Therefore try to find a reference card.
Also, if you want a guaranteed overclock, you can try the overclocked versions which is pretty much plug and play. Each card is different so if you get a reference designed card, don't be disappointed with results. It's the world we live in!