Best voice acting ever? Vampire the Masquerade: Bloodlines. Everything is voiced, even the new lines added to characters you haven't visited for awhile after you do certain key things. I mean, these avatars are sitting in these tucked-away places you'd probably never go back to, and on chance you go back and talk to them, and they have new lines voiced as professionally as the main quest ones. And the cast from that game is pretty much all professional voice actors who know what they're doing, and it definitely shows. The dialogue is actually the main reason most people are still playing the crap out of this old, under-the-radar game.
Worst voice acting ever? Everything out of Japan before the late 90s was terrible, so forget specifics. Can I nominate the Sims? Is that fair? That lunatic babble certainly curdles my blood.