For maximum security & privacy, go with GrapheneOS (used to be CopperheadOS back in the day). Caveat is that you won't have Google Play (or even MicroG), which means some Google apps (and paid apps) won't work and if you want apps from Google Play then you need to install Aurora app.
If you want the above, but also want access to Google Play apps, then install CalyxOS.
The third option is LineageOS (was originally CyanogenMod back in the day). This custom ROM is the most accessible for a variety of devices. It's good for privacy, but, because the bootloader is left unlocked (which may or may not be relevant depending on your threat model), it is the least secure of the three.
Both GrapheneOS and CalyxOS have very user-friendly installation methods, but exist primarily for the Pixel line of devices. LineageOS has the most involved installation process, but it's available for the widest variety of Android devices.
I've installed all three ROMs on several different devices. For the average person, CalyxOS will probably be the best bet (though I think the Trebuchet launcher in LineageOS is better).