Some of these might have already been mentioned, but here goes anyway:
1) The entire script of Resident Evil 1. It's just.... priceless. Watch the cutscenes on the internet - you won't believe how bad the voice acting was back then. To be fair, it was bad in most other games too
2) Mass Effect "love scene" dialogue. As much as I like Mass Effect, as good as the voice actors are, that scene was full of bad lines, and the voice actors almost sounded embarrased to say them. It was just awful
3) SHOW ME YOUR RAAAAaaaaAAAAAAaaaaAAAAAAAAGE!!!! That speaks for itself. I'm a MGS fan too, but no matter how you say that line, it was just........ bad.
4)Anything Drebin said in MGS4. Double so his bits of dialogue after the B&B crew.
5)Okay..... just..... Metal Gear.
Now - I do like the Metal Gear Series. Especially MGS3 and MGS4. I do like the plot and I do find the characters interesting, and Snake, Otacon and Campbell usually have great lines. But for some reason, Kojima feels like he MUST insert random, stupid and juvenille jokes and lines into his games. The entire B&B thing was just stupid - hell just saying "beauty and the beast" crew sounds idiotic. Anything Fortune said in MGS2 was waaaaaaaaaaaaay too melodramatic and overblown (I was half expecting a theatre stage spot light to shine down on her while she moaned endlessly about her sad, sad life). And not to mention in MGS3, all the villains had outrageously silly speeches (especially The Fury - cool concept and design, ridiculous speech and backstory). And "la li lu le lo"? Pure pain whenever I heard it.
And of course, the single most embarrassing line of dialogue from the Metal Gear Series:
"I live on!!....... THROUGH THIS ARM!!!"
No Kojima. No. I know he "explained it away" in MGS4 - but still...... no. When you are trying to get people interested in a game series, you do not want them to watch that scene.