For me, during my current rewatch, its when she sentenced Tuvix to death. That was brutal.
Especially considering its documented that the transporter can duplicate people - they couldn't have just duplicated Tuvix before separating him? Ryker's 'brother' gets a pass, but Tuvix doesn't?
Also, she definitely shows quite a bit of favoritism for certain characters. Its the main ones, and I realize its just a show and thats how it needs to be, but its so damn blatant about it. Chakotay is traveling with a female ensign in a shuttle and she is killed, and no-one, for the rest of that episode, so much as mention's her. When Chakotay informs the ship of it, Janeway literally ignores it and asks him how HE is. Like the woman was just some nameless piece of garbage that can be thrown away. When Kes or Neelix are in danger Janeway wrings her hands and is willing to go to the ends of the Delta quadrant for them, but an actual, STARFLEET crewmember? They're trash. And if there is the slightest chance an evil Caradassian woman had Chakotay's baby, Janeway is willing to go weeks out of their way to help him (which was a trap, which they suspected, and walked right into anyway because she has no control over her emotions).
Sure, its ST, and I have to admit Voyager is probably a lot more like the TOS than most of the other shows (planet/species of the week), but yeesh, they can be a little less blatant about it. 'Red shirts' indeed.