The best replacement for x16 is x8.

The best alternatives to x16 are rated as following:
★★★★★  x8
★★★★★  pci express x1
★★★★  x18
★★★★  x4
★★★★  dual x8
★★★★  x8 sli
★★★  x1
★★★  pci express 2.0
★★  x8 pcie
★★  pcie x1
★★  x8 crossfire
★★  pci express x16
★★  dual x8)
★★  trilinear
★  agp
★  pcie 2.0
★  pci express 3.0 x16
★  x8 x8
★  pci express 2
★  agp 8x
★  pci e x8
★  3.0
★  pci express
★  pcie 3.0
★  pci express 3.0
  pci express 3
  pci express 2.1
  x12
  pci express 2.0 x16
  pci express 2.0 x 16
  sli x8
  pci-e 2.0
  pci
  pci express 2.1 x16
  pci 2.0
  pci express x16 2
  pci x16

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Speaking about x16, one probably shouldn't forget about x8 pci express.

Other search terms related to x16 and their ratings are:
★★★★★ x8 pci express
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x16
x8
pci express x1