The worst voice acting I ever heard in a game was in some fan-made Star Wars mod.
The second worst voice acting I ever heard in a game is AquaNox.
It's not all bad, granted - some characters sound okay. Most of them, though, are just reading their lines with no emotion to speak of, sometimes interrupted by equally emotionless laughter. After a while (say, half a minute) it really gets on your nerves, ha, ha, ha.
The writing of the dialogues is equally horrendous. No one ever talks like this, thank goodness.
The intro is possibly the worst offender - the main character talks and talks (possibly explaining what happened in the first game), and it sounds like he's reading a shopping list. Forever.
And then you get to float around and shoot stuff.
Fascinating.
Caveat emptor.