Matt Burns at TechCrunch:
“The additional horsepower isn’t needed for general use, but the added graphics cards supercharge Macs for VR rendering and gaming. Only a handful of eGPUs are compatible with macOS so choose carefully before adding one to your rig.”
The AMD Radeon RX 580 is the best card for games on the compatibility list, but most of the eGPU boxes are pretty bulky. Your best best is probably the Sonnet Box, which is the smallest and comes with an RX 570 built in (with a cheaper RX 560 option). The caveat is that it doesn’t have USB-C output, so you can’t put it between a Mac and the LG UltraFine monitors that Apple actually sells. Other eGPU boxes have this feature.
This will probably go unheralded but I think it’s a huge milestone in Mac history: serious game support! Finally!