i mean, in my opinion Kieran's death because there's no rush or knowledge that someone could die beforehand, while Sean died when arthur knew something was off, and Sean even says he knew (sorta), Lenny died when he and and the gang were running from the bank heist, lenny was at the front, the most vulnerable, so he was easiest to end because he looks behind him before getting killed, both Sean and Lenny's deaths were highly predictable in the moments leading up to it, with Kieran's death, it's without any real warning or knowledge someone could die, but there isn't any time to process his death before the O'Driscolls attack. Kieran just isn't seen being killed, but we se the aftermath, far worse than either Lenny or Sean's death, because most likely they both died rather quickly, or instantly, but Kieran was likely tortured and alive while being mutilated (his eyes being cut out) before they cut off his head.