1. The most common Serial to USB modules on development boards
Many makers have purchased development boards such as Arduino UNO, STM32, RPI Pico etc. If you need wireless connectivity, then you might even have bought ESP32, Arduino Nano 33 IoT and Ameba RTL8722 or BW16. Almost all of them have come with a on-board Serial to USB module that is one of the 3 ICs shown down below,
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CH340
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CP2102
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FT232
Serial to USB module is so useful because it provide a easy and stable way to interact with host PC and even upload new firmware onto microcontroller through it.
2. Why does it matter which one is the best?
I knew very little about these ICs until I decided to design and make my own Dual-band WiFi + BLE5.0 microcontroller early this year.
The idea is simple, I want to see how difficult it is to DIY a customized IoT development board using off-the-shelf BW16 module that is based on Realtel’s RTL8720DN SoC.
The fact is, I succeeded!
It comes in 2 versions,
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With breadboard
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With DIY PCB
Majority of the circuit design is referenced from @Alphi_Jiang 's design, and the software obstacle was also overcome thanks to the community’s contribution.
Though my DIY dev. board project is still ongoing, but if you are interested, you may check out this forum where I will post my progress and updates 
So back to the topic,
Why still talk about Serial to USB module?
Because this to me is one of the “challenging” part of my project, as I expect the Serial to USB module to serve 2 purposes,
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Provide UART interface between PC and MCU
This is a must because with that only can I read log, key in AT command, or even upload new firmware onto it -
Supply power to the MCU
Since it’s important to keep the circuit simple and cost-down, I expect to use this module to supply power to the RTL8720DN IoT module.
The real battle starts here
In fact, all these 3 Serial to USB module can do what I expected, but with different specs.
I read through their datasheets, CH340, CP2102 and FT232, and here is what I found,
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Goal 1: To provide UART interface between PC and MCU
Most critical part here is which logic level can they run on and which one has the highest Baud Rate,
Model Logic level CH340 3.3V & 5V CP2102 3.3V only FT232 3.3V & 5V Model Baud Rate (bps) CH340 2,000,000 CP2102 1,000,000 FT232 3,000,000 It’s clear that FT232 is the winner
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Goal 2: To supply power to the MCU
With all the ICs rated at 3.3V, the one that can draw highest current will win,
Model Output current from Vcc (mA) CH340 20 CP2102 26 FT232 50 It’s obvious that FT232 is the winner again
Other things to notice
All these Serial to USB modules are NOT designed to supply constant and stable power to external device, thus don’t expect them to work as per normal when you hook your DIY development board with heavy loadings or using power-consuming peripherals such as WiFi, LCD or Camera.
Also, in my testings, my DIY development board would reset itself when there is a surge of current drawn. To counter this, you may use a big capacitor(220uF) across the Vcc and GND to supplement the current when the surge comes.
The Conclusion
With the right source and timing, all these can be bought at the same price.
Therefore, to me
FT232 is my GO-TO option, and I have already used it in my project
PS: as a software engineer, I might have made mistakes I myself couldn’t notice, so please forgive my amateurishness and let me know what you think 
Hope this is useful to you~
Stay healthy and happy coding 