Is search engine optimization a concern here?
I'd choose copy-paste for the whole process of copying/cutting and pasting text around, because that's one everybody (both beginners and advanced users) would understand. Tags shouldn't trigger questions.
Another reason is that it could happen that
yank are not the only way, or not the more natural, to achieve the wanted action (for example rectangle regions, CUA functions...).
Use copy-paste, which is the common layman term, as the main tag. While Emacs reserves paste to mean pasting from some operating system clipboard outside Emacs, copy-paste is the most discoverable tag for most users. The menu nowadays calls the commands Cut, Copy and Paste.
Some questions may warrant more specialized tags, such as kill-ring for questions about manipulating the kill ring (e.g. select from menu). Interacting with the OS copy-paste functionality is one such subtopic; there are currently two (1 2) questions about this, tagged clipboard. I'm not sure if that's the right name, as the difference between copy-paste and clipboard isn't really apparent.
I would say copy-kill-yank, because it is this how Emacs calls them.