AI Hacking = basically useless, if only because it gets stopped by armor/shields. Cryo Ammo is also less than stellar.
I use:
Squad Disruptor Ammo
Assassination Cloak
Incineration Blast
Agent/Assassin (either is good)
Heavy Warp Ammo
Typically, I bring along Mordin and Miranda for their good debuffs. Squad Disruptor Ammo is obviously for the large groups of synthetics typically encountered, or for the shields of the YMIR mechs, which even the Widow has some trouble with. Assassination Cloak is a no-brainer, in my opinion. The Widow is so effective at taking out armored enemies, that Heavy Incinerate is not really necessary; Incineration Blast works surprisingly well against Husk hordes and acts as the Infiltrator's sole CC ability. I personally don't see a huge difference between Agent and Assassin, so I think either will work just fine. Heavy Warp Ammo is what I use most of the time, because it provides a 50% damage bonus to everything except shields, which Disruptor Ammo can handle.