Question "Best" ROM for putting on phones to ship?

Though, I understand how most of the processes work, I am new to this whole Android-ROM topic.

I am wondering what in your opinion the "best" ROM is, if I intend to put it onto real devices on the phone factory side before shipping them to customers all over the world?

I know the term "best" is very arbitrary and there is not a one-line answer to my question. Heck, the question itself is challenging However, I would like to start a discussion on this and just get some experts views on this. It would be really great to hear what you would suggest and especially why.