Here is a discussion about whether to use ご返事 and お返事, but there seems to be diverging opinions:
ご返事:
お返事:
Given that I always write in the context of business (sometimes about my reply, sometimes about my client's reply), can I stick to always using ご返事 ?
ご返事 sounds ungrammatical to me. If it is in use, then that may be an idiolect/dialect. But I suspect that, for the majority of people, the grammatical form is お返事. As I read your link, I feel that ご返事 arose as hypercorrection. Some people believe that the choice between お vs. ご prefixes is strictly correlated to whether the word is a native Japanese word or a Chinese origin word. This holds as a tendency, but to say that it strictly holds is wrong. Nevertheless, people who believe this tries to put ご to 返事 because it is onyomi.
The correct answer is that both お and ご are correct.
This is a very rare case. The reason is that in the past 返事 was not always read as へんじ.
It was at one time かえりごと.
So conforming to the normal patｔern you had お[返事]{かえりごと}.
However over time it became more and more common for it to be read as へんじ so you had the normal pattern of ご[返事]{へんじ} also.
Today 返事 is always read as へんじ however the use of both お and ご still persists.
First, we need to make a distinction between 返事 as a noun, and 返事する as a verb
1.
When it's a noun, as in
お／ご返事をお待ちしております
the お or ご create 美化語.
2.
When it's a verb, as in
お／ご返事いたします （humble, 謙譲語）
お／ご返事下さい （respectful, 尊敬語）
the お or ご create 謙譲語 or 尊敬語.
As for the question of whether to use お or ご, I think it's a matter of personal preference, but I think there is a general concensus that ご sounds a bit more formal than お. For example, in non-敬語 speech, ご返事 (as 美化語) sounds awkward
? ご返事待ってるね ← not wrong, but quite awkward
○ お返事待ってるね ← natural
I cannot imagine that anyone would be offended by お返事, so if I had to choose, I would stick to お返事.
