The rules of this game aren't entirely clear to me.
When the war is over, are you STILL the T-1000? You you still have it's emotionless countenance? Are you stuck in that time period?
It seems to me that the best answer has to be the one that causes the least amount of harm to humanity (or perhaps has the best positive impact, but that'd be even tricker). Why? Because a do-no-harm approach would make me happy currently. And I think we can discount anthropomorphizing the T-1000, and ignore what we think "I'd" want if "I" was it.
Why? Because it is a machine, and thus will likely not gain any emotional value from killing, as the Terminator series has led me to believe that the T-1000 has no emotions.
"If by feelings you mean emotions, I’m pretty sure you still don’t have any of those." - John Connor
Really, the question is now "What time period could you equip an army with a ruthless, psychopathic killing machine and do the least amount of harm."
So, what war would best help humanity? Hmmm. Nix on the Vietnam war, Iraq war (either), Afghanistan war (either), even the Korean war. All those are horrible decisions. Tons of men and women and children would be killed for doing nothing more that (from their perspective) protecting their homeland. And either side would then have an unstoppable killing machine, and that would be some seriously corrupting power.
WWII I'd stay away from too. Too much killing, and any significant change to that War early on might have seriously effected the Cold War - Increasing the chances that we didn't avoid nuclear annihilation. WWI as well, too much killing for what was really every European nation's collective fault.
Ancient wars would be a total slaughter, which would be horrendous. But this might be our best bet, if we can find a war on a remote island, where the T-1000 might be "forgotten" for long enough? Fuck, how the hell is the world going to rid themselves of this thing?
Lets go with: the ritualized warfare that was taking place on the Marquesas Islands at the earliest point before the island was found by western explorers.