Custom of course! But only if optimized and stable!

Customized rom are better usually but, not always. I think this question is not a right question to be ask because, custom roms are not always better than stock one for example some of custom roms bring many more futures than stock android but, their heavy UI or apps or ... make it laggy and unenjoyable. Sometimes they are very complicated and not suitable for everybody and sometimes they have their pros and weaknesses compare to stock one which is not a welcome feelings. So, being better or worse depends on which custom rom we compare to the stock one.

Custom roms ui and features with stock like performance

I like stock Android..

Custom os provides tons of feature where as stock android there is minimal options
Custom os is best.

Custom roms
Got more features then stock

customized rom by meizu.

I prefer customized ROM because it customizable and its freedom to do more.