So many...
Anything and everything Duke Nukem says, especially: «Your face, your ass... what's the difference..."»
Many of the units' responses in Red Alert 2, in particular from the Allied forces: The SEAL special unit «Little C4 knockin' at your door!...» or the Chrono trooper when he annihilates an enemy unit from existence «Never existed...» or from Yuri, when he mind-controls an enemy unit, saying «A new friend joins us»... and many others.
Lots of the great dialog/writing from the Half-Life series, some of which from the G-Man, including: «Prepare for unforeseen consequences.»
In BioShock, Atlas' «Would you kindly?»
In Diablo II, the Blacksmith: «I shall make weapons from your bones!»... or Diablo himself when he spawns: «Not even death can save you from me...»
Many of the one-liners and responses from all classes in Team Fortress 2, including my personal favorite being our beloved Heavy Weapons Guy's «Doh doh doooh! doooh! doh doh doh doh come sing wiz me! doh doh doh doooh doooh!»
One of my all-time favorite video game series for good one-liners and quotes in general, namely the Warhammer 40,000 series (including Dawn of War II, also see my avatar right now). I would have hundreds to name from those games specifically, some of which are: «Only in death does duty end»... or the Librarian's «Hope is the first step on the road to disappointment»... or the famous Force Commander's «Walk softly, and carry a big gun.»... or basically the entire dialog at the battle between Gabriel and Isador (in DoW1's campaign), very memorable.
But honestly there would be literally thousands of one-liners and various quotes I could find, there's just too many, from games of the late 1980's that I grew up with passing by the golden 1990's to the memorable 2000's, on Consoles and PC alike, I know right now I'm forgetting some of the best ones from my personal collection of games, and beyond. I'll just keep reading what others will add, I like threads like this (even though there's already dozens of results from Google if you try it yourselves), I'm plunging memory-first back in time and nourishing my nostalgia with this.