there is no "Best ROM", you must find the one you like the most :man-shrugging:
Whilst there is no "Best ROM" as mentioned above, you will have to find one which suits you and which you enjoy using, however, here are some ROMs that I personally recommend:
LineageOS is a custom distribution of Android designed for full freedom, security, and for longevity of various different devices.
Lineage first appeared after the popular CyanogenMod project had died off in 2016, and is now one of the most stable and polished ROMs out there. The verification process for official devices is thorough and guarantees a smooth experience, and for every official device there is detailed install instructions starting from unlocking the device's bootloader.
ArrowOS is a custom ROM aimed at "Keeping things simple, clean, and neat.", as they call it on their website.
They base from AOSP, and not LineageOS like some other projects, and add only features which they think will be useful to the end user, whilst also improving general performance and fluidity, as well as battery life.
Whilst many don't like DotOS, I consider it a pretty solid Custom ROM overall.
DotOS stands for "Droid on Time OS", it aims to stick to the "Simple, Unique, Secure" philosophy.
They base themselves off of AOSP, and then change the design drastically from what is shipped on Stock Android.