There is none of an endorsed abbreviation for the Bitcointalk Forum officially.
Theymos too doesn't use any of those mostly as stated here.
You can use whatever you want...
I almost always call this site "the forum", "the Bitcoin Forum", or "bitcointalk.org" (spelled out completely). I very rarely call it just "BitcoinTalk," so any abbreviation of that name isn't natural to me. I don't see where people get BTT from, though.
Still, I would go with
Bit
Coin
Talk -
BCT, which looks most logical way too me.