I’ve gone for Attack. Although the Cybermen aren’t perfect in it (they display too much vulnerability early on and get shot up rather easily), it’s the last time they are presented as genuinely sadistic and menacing before being turned into an absolute joke. They’re crap in every story after that, save perhaps World Enough and Time (and even then, it’s so bound by the Missy rubbish and an admittedly lacklustre second part where they become laughable cannon fodder again).
Silver Nemesis is fun, but objectively rather crap (and one of the worst McCoys), and the Cybermen are bloody useless in it bar the action scene kicking off episode 2(although it is admittedly amusing that jazz music fucks them up). They are awful in every New Who appearance after that, bar the series 10 finale, and are cheapened into robotic parodies which either spout crap “delete” catchphrases, get blown up by the bloody power of love, start flying like fucking Iron Man whilst being enslaved by the parody Missy character by Death In Heaven, and with none of the eeriness, power, brutality or dignity of their former selves.