As Higgs mentioned
Calibre is a Library
Manager, Why not let it do its job... Manage your Library.
For most reader devices,
Send to device is the simplest way. Each device (series) has a preferred format setting (usually, the default configuration is good, but if you have had Calibre installed for some time and later upgrade your model, you might revisit what is now the best. eg older Kindles:Mobi. Newer Kindles: AZW3)

Original format. IMHO Keep it (and note what it was in a custom column). You are never going to change readers? What if it uses a different format?

Conversions LOSE finer details (formatting). Converting a conversion speeds that up

Converting from the Original in most cases, leads to the best results.