Best no scope action?
I'll say 'Every headshot I made in the original UT.'
People wanted to play snipers-only for so long I just became a brain-surgeon.
That, and UT's sniper rifle is unquestionably the most broken sniper rifle in just about any shooter ever. Hitscan weapon, 50 shots, near-semi-auto firing rate, zero recoil (being UT, of course), one of the best variable scopes ever, and of course, laser-precision when not scoped.
Combine that with surprisingly forgiving headshot hit detection and you have a broken-ass weapon.
Its only contenders were the Shock Rifle and Flak Cannon (and circumstantially, Rocket Launcher).