I greatly prefer top posting because it emphasizes what I'm looking for in a email reply which is new information. Moreover, modern email clients automatically hide the quoted text so only the reply, with the new information, is visible but context is available if necessary.
For example:
---
Let's meet at 8 pm.
(Show more from person A).
---
Typically, the context will be clear from reading the reply and the sender. If not, I can establish context by clicking on the link:
---
Let's meet at 8 pm.
> Agreed, when should we meet?
>> I think we should meet face-to-face to discuss this.
>>> This is a super-important discussion.
>>>> I propose to implement HTML mails.
>>>>> Plaintext mails suck. I missed important information because the mail used asterisks instead of bold text.
---
With top posting, I can stop reading the previous mails as soon as I understand what's going on. Consider the opposite with bottom posting: Here I would have to read the entire context from the beginning or am at the mercy of how other people edited the context.
EDIT: Put --- on individual lines. Fixed asterisks formatting.