What is the best PCIe GPU?

An x4 PCIe device has a theoretical max transfer rate of MBsec according to wiki

For example, a PCIe x1 slot provides one lane and transmits data at 1 bit per cycle x cards seamlessly plug into PCIe slots, and

pcie slot size This video explains everything you need to know about PCIe slots, including PCIe slot Old

pcie2 slot A single PCIe slot can now support x16, x8, x4, x2, or x1 lanes For example, PCIe x4 indicates a Gen 3 expansion slotcard with four lanes