What USB to UART chip provides highest data rates with Arduino boards?
Here's my experience with the above chips over the last two years:

FT232R is the fastest, works well with any baudrates. UART adapters with it are sometimes more expensive than alternatives.

PL2303 is good if you don't plan to go faster than 115200. Additionally, it has the strongest 3.3V reference (available on some adapters as 3.3V output) which can be abused as a 3.3V 20mA power supply. Other chips will refuse to work if you draw more than 5mA from the 3.3V reference.

Prolific is currently fighting against clones by making their up-to-date drivers incompatible with non-original chips, so if your adapter won't work in Windows with code 10, take a look at this page.

CP2102 which looks great in theory didn't work well for me (in Windows 10, with driver v10.1.3). Bulk data transfers at 115200 were in some cases 4 times as slow compared to other chips.

CH340G has a sketchy-looking windows driver, but otherwise works quite well and is often the cheapest option available. Unfortunately, it lack the intermediate baud rates, and getting 2Mbps UART link to work may be hard sometimes. Arduino clones which have this chip on-board do work at 2Mbps without issues (the ones I have seen do, anyway).