MediaWiki:Discussiontools-defaultsummary-reply defaults to "Reply", which is generally fine.

I personally prefer "replying" instead of "Reply", as the latter looks like an order to "Reply!" in my mind. Other users might prefer "answering", "response", "2¢" or "re".

Is it worth making this a user choice? Replying to discussion messages is a pretty common action, and there is probably a noticeable diversity in edit summary preferences when doing so.