At forums.starwars.com my favourite forum by far is the BC&T forum.

My favourite thread used to be the Welcome some BCaT VIPs thread, but it has been locked. It still has a lot of interesting information in it, so hopefully it will avoid the usual locked thread purges.

There are also a couple of threads that are pinned to the top of the boards that for some reason often go unnoticed: the Forum FAQ and Thread Directory and the BCaT Questions threads, designed to reduce the clutter in the forum.

My favourite discussion threads are:

EU leadup to Revenge of the Sith

Star Wars: Dark Nest (speculation)

Legacy of the Force: Betrayal (speculation)

Outbound Flight (speculation)

and a couple of older ones to which the authors themselves contribute:

Destiny's Way (spoilers)

and

Republic Commando: Hard Contact (spoilers)