Hm... I'd have a terminator bodyguard either King Louis XVI or Nicholas II of Russia, or kill FDR or Lincoln before either got elected.
Bodyguarding Nicholas II prevents the complete communist takeover of Russia (best case scenario the communists are crushed, worst case Russia splits into two), saving hundreds of millions of people from being killed by communism. Cold War is averted (if Russia splits into two then the communist bloc won't be anywhere near as powerful as OTL's Soviet Union, minimizing their influence). Russia and the Allies will more closely coordinate against the Nazis, bringing a quicker end to WW2 and a more peaceful post-war period. Overall a happier timeline.
Bodyguarding King Louis XVI gives France a better chance of not becoming a crazy totalitarian empire and saves tens of millions of people. Also diminishes the chance of communism happening which can have huge butterfly effects.
Killing FDR before he gets elected means that FDR doesn't get a chance to provoke a war with Japan over greed. However, by this point Japan has still been pretty much taken over by a cabal of war cultists and they're going to cause a lot of headache in the Far East, so an earlier change in the timeline may be necessary.
Killing Lincoln before he gets elected means that there is hope for a President to be elected who will resolve the national crisis more peacefully. If he does what the British did and promises to buy out the South's slaves so that Southern landholders don't just straight up lose their wealth and get a chance to transition to other sources of income, then the Civil War will be averted. Granted, there will be a few "yes really" racist shitbags who will want to retain their slaves, but I'd imagine that the vast majority of the slave holders would rather accept a fat sack of cash rather than try to wage war, and without the support of that majority, the minority of racist shitbags will have little choice but to go along with it as well. With no Civil War, the US is unlikely to become an Imperialistic Empire and start going into a conquest spree like they did in OTL. The US won't vassalize Japan and teach it Imperialism, saving a lot of headache later on.
So for me it's between killing Lincoln or bodyguarding Nicholas.
Killing people who had a major impact on history just throws everything into disarray in massively unpredictable ways. Like what happens if you assassinate Hitler, and if so at what time? Europe was ripe for another war but Hitler wasn't some figurehead.
Hitler was a problem yes but he was a symptom of a great problem, one that could have been avoided if WW1 had not happened and Germany wasn't wrecked. Bodyguarding the Archduke could possibly prevent disaster, but there is a good chance everyone would be fighting each other anyway because everyone was allied with each other, and it just takes one incident for everyone to be suddenly at war with each other.