Best, or best supported, USB-Serial adaptor for OSX
I have two Prolific USB-to-serial adapters but they are made by different Vendors (one is Prolific, the other ATEN).

I've used their supplied drivers just fine. Note that there are open source drivers for these devices too available here: https://github.com/failberg/osx-pl2303 FYI my use case is actually patching them through to a VirtualBox Windows VM and using them in Windows, so I have two layers of drivers and have not had any problems yet.

However, I think the best supported USB-to-serial devices are those made by Belkin. This is subjective, but I've used them for years without problem, and they are a large company with a reputation. They will likely be around in the future.