Until recently, my answer to this question would have been Saints Row 2. I played the PC port of the game on Steam, and it contained its share of problematic glitches. In fact, more than once I found the game to be completely unplayable. Worst was when the screen would randomly blackout in the middle of a mission with the sound still playing in the background. It only happened a handful of times, but even one occurrence of such a glitch is one too many.
But as glitch-ridden as Saints Row 2 was, it doesn’t hold a candle to Fallout: New Vegas.