Twilight of the Armor is definitely the best.  Are you willing to wage $20 on a game that's like 12 years old?   I see Galciv2 Ultimate retailing at $20.  If so, I think grabbing all the DLC and playing the campaigns makes sense.  I found the Dread Lords campaign very fun; but you need to amp up the difficulty.  Be prepared for a different, weird econony.  Lots of strategy guides out there.

However, if this is just to bide your time while Galciv3 gets worked out, I don't think we can reliably plan on Galciv3 ever having all loose ends tied.  I chose to forego being a Galciv3 founder and just now bought Galciv3+Crusade for precisely that same reason, and that hasn't quite worked out as I had planned. 

2 is very good and definitely worth playing, even now. It recently had a fan patch added to the main game which rebalances virtually everything.

I see no reason to play a game you aren't already thrilled about.

Couldnt agree more. GalCiv pre-crusade was satisfyingly playable, from bug/balance point of view. Of course and obviously, when new additions like espionage and others will be done properly, the gameplay will be richer than it was pre-crusade. However, there aint much choice right now and I suggest to stick to pre-crusade game.