When comparing Rename Master vs Flexible Renamer, the Slant community recommends Rename Master for most people. In the question“What are the best bulk rename tools for Windows?” Rename Master is ranked 3rd while Flexible Renamer is ranked 9th.
Specs
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Pros
Pro It supports EXIF and ID 3 tags, scripts and regular expressions, and does not require installation
Pro Powerful but easy to use
It has many options, including renaming using tags (ID3, Exif...), can number files, use wildcards or regular expressions, walk a hierarchy of folders.
And it has presets for common renaming tasks, like removing parentheses, "Copy (n)" or "Shortcut of", and so on. Its preview feature avoids errors, and warns (a bit late though) of naming conflicts.
Pro Free
Freeware and lightweight (archive is less than 2 MB).
No install, but might need removing options of Explorer integration / scripting if removing.
Pro Context menu in Explorer
Can add a context menu to Windows Explorer to rename files in a chosen folder.
Pro Tag management
Can rename using ID3 (music), Exif (photo), IPTC (images), DOC or HTML tags.
Pro Has many translations
Available in English, Japanese, French, Spanish and German.
Pro Scriptable
You can write scripts in VBScript or in JScript (native Windows scripting languages) for complex renaming operations.
Cons
Con Seems to be no longer maintained
Last updated in 2011 (v. 8.4). Yet it works fine in Windows 10 and still does a tremendous job.