I have to agree with a few others that Bulk Rename Utility
is both the most powerful and easiest to use (for me). The UI is cluttered for good reason — it's all right there in front of you, and once you stare at it for a couple of minutes it makes sense (each function is labeled and numbered). No tabs, no backdoors, no subdialogs, just one level, and BAM! you're done. Makes it easy to save any renaming profile, and it's donationware, comes in Unicode, and 64-bit versions.
I considered and then rejected doing a meta-review of renamers a year ago because there were so many, several of them very, very good, and most of them useful. But their UI's varied so much among each other, I soon saw it became a matter of personal preference; often, simply what you were accustomed to using. Best thing is, the high-end file managers like XYplorer and DOpus include extensive renaming abilities if you don't use one of these utilities.