Best clipboard manager for X ? (edited)

CopyQ

I use parcellite. Sensible UI, has all the functionality I need and it's not tied to any desktop environment.
The functionality I care about:

• history, including customizable persistent entries
• customizable key bindings
• handles both primary and selection and can sync them
• can trim whitespace
• can launch customizable actions on the active entry

Selection buffer.

Unless you mean clipboard manager, in which case it'll depend on your desktop environment.

Copyq

If you're using gnome there's a really nice extension.

I like greenclip. Pretty minimal, makes a solid clipboard manager combined with dmenu (or rofi)