What determines which reply is shown under the original post in the discussion list?

RyanSteele-CoV 

So the answer to this is two fold, if you are referring to the order of the replies under the topic (on the topic page) then that is controlled by the setting that Deleted and SergeiBaklan were discussing. Note: Threaded doesn't work.

If your referring to the 'first reply' view you get on the board or the category level i.e. 

This is either:

1. The accepted solution
2. The most recent reply at level 1 of the conversation
3. a pinned reply

Except.. (there is always one isn't there).. this view is also cached for anywhere between 1-30 minutes depending on if your logged in, when it was last updated in the server side cache etc etc..

I don't especially like this inconsistent behavior and its something high on my list of 'to do better' when we finally release the newer version of the platform later this year.