Best OS to use for MacBook Air 2019

Do note, if you are still running Mojave, you probably still have 32 bit apps that won't work on any version past it.

Make sure you have replacements or updates ready for those Apps.

click here ➜ 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

and here ➜ https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

A 2019 MacBook Air can update all the way to to the current macOS 14 Sonoma. It is always recommended to have the most current macOS version your computer supports, to have the most up to date security fixes and patches and the newest features.

Ventura might be a somewhat safer choice than Sonoma – there's been more time to shake out out any problems with it; more time for application vendors to fix any compatibility problems with their applications.

If you need the latest and greatest features and security fixes, those would be in Sonoma.

This assumes that you are not heavily tied to 32-bit applications – that you can upgrade, replace, or stop using the ones you are using now. (They won't run on Catalina or later.)

It isn't really an issue of which OS works better. They all work fine. It is good to normally install the latest offered update or upgrade that your Mac can accept. Doing so often gives you added features and increased built-in security.