Cersei Lannister took part in the death of Robert Baratheon. Aside from the other references in the show and books, she specifically "confesses" this to Septa Unella during the wineboarding scene. There's no thought of argument otherwise. Arguably, all of Ned's men in the throne room can be tied to her, since she bought Janos Slynt to have him doublecross Ned.
I don't know if you want to count where she was circumstantially stopped from being responsible for the deaths of many people along the way, depends if you're more concerned about fact or intent I guess:
- She orders Ilyn Payne to be at the Red Keep during the siege to spare all the ladies from rape (by killing them first).
- She was ready to poison Tommen to prevent his capture by Stannis Baratheon.
- She wanted Tyrion dead, and sought to push that when provided with an opportunity; although you could argue that was "justified" if she genuinely believed him responsible, it is heavily alluded to that she has wanted Tyrion dead for years.
- If you count direwolves, she pushes for Lady's death in retribution for an incident Lady was never involved in.
In the books, there are other characters that she effectively gets rid of or orders gotten rid off along the way, and other plots. The second High Septon (on the basis that he was appointed by Tyrion) was Kettleblacked, her maid Senelle was given to Qyburn for spying on her, Falyse Stokeworth was given to Qyburn after the Stokeworths failed to contain/kill Bronn, etc. None of these characters are part of the show, however. She publicly forbids Robert from being in the melee at the Hand's Tourney knowing that will convince him to do so (and so have him "accidentally" killed). She sends Balon Swann to Dorne with orders to assassinate Prince Trystane and blame it on Tyrion.