What is best practise for a simple forum thread?
Unordered List. No doubt about it...
It's a list of topics... A list of responses... A list of things people are saying... However you look at it, it is a list.
Have a peek at this previous thread:
A list. A forum thread is a listing of messages and it should be interpreted as a list, not as a table, tables are for tabular data, and tabular data alone, div's should be out of the question by default
Forum threads aren't tabular data, so they shouldn't be a table.
I prefer lists, since that's a fairly good semantic fit. You also have the added benefit of styling flexibility. It's easy to make a list act like a table. It's harder to make a table act like a list.
Whether you use ordered or unordered lists is up to you. Ordered lists might be a good fit, if you consider them to be a chronological list of responses to the thread topic...
I'm pretty sure you shouldn't use tables, but after that, it's probably your call.
I'd go for a few
<div class='message'> tags after each other.
I think depends. What is a simple forum thread? A list of topics without more information? Then go ahead and use lists, but if you show topic, author and date (is a simple structure yet), or only topic and author, you get tabular data, so a table is more semantic.