I would start with /dnc tbh. It brings utility that youre going to wish you had when youre lowmanning(pretty common in aby) and soloing. Stun, haste, heals and stealth. The stealth alone will be helpful, since as a new player youre not going to want to spend gil on oils/powders, and running around aby means stealthing a lot.
/war is great but not all that effective until you gear up and get the good atmas. You can /war and still be useful, but youll probably die a lot more. You may not always have a whm around.
I guess the real answer is, level all subs lol. But if it were me, id take care of dnc first.
Edited, Jul 30th 2012 5:11pm by Llester