People will choose the brand they own or have experience with as the "best". Really no one buys the same GPU with different manufactures often enough to make a call. With that said manufacturing partners (power color, sapphire and such) will do things to stand out. They will do such things as include beefier cooling, "cherry pick" the memory and GPUs as well as use higher quality capacitors for overclocking purposes.
For the record my first GPU purchases were with powercolor. The cards were fantastic.