Best SSD Thunderbolt external hard drive for 4K video production?

I could not really say what is best.

Although theoretically a Thunderbolt 3 SSD may give you the greatest throughput, I wonder if the extra cost will be worth it. There are several SSD that connect via USB-C and can give a throughput of 900+MBps. That is ample even for several 4K angles.

I have edited 4K material from a Crucial X8 and there is no hiccups. Theoretically the internal drive on my 16” MBP may be three times faster but the external is plenty fast that there is no discernible difference in practice. Unless you plan on working with 4K multicam with a lot of different angles, you may not see the benefits of true Thunderbolt.

Just to add to what Luis said, you'll probably be fine with 4K H.264 but maybe not if you're using ProRes 4444 or RED RAW or some 10-bit video formats.