It's unclear if Missandei's beheading (done after Dany refused to surrender to Cersei) was an opportune convenience or the plan that Euron had all along. Either way, her death (the death of the last prominent woman of color on the show) felt especially cruel and mostly pointless. Dany was never going to surrender no matter who was up on that ledge, and if Cersei's team knew enough about Dany to know who Missandei was, they would know that too.
That said, Missandei's last word of "Dracarys" will hopefully be the push her queen needs to burn Cersei to the ground, and so we're giving her a high spot on this list for now.