I play mostly mainstream titles these days, so I have a limited scope but:
Best: GTA 4 had mind-blowing voice acting, and hours upon hours of it. KoToR was also exceptional.
Worst: Oblivion, for the sheer repetitive stiffness of the 4 voices used among hundreds of characters. Starting out with Patrick Stewart only raised everyone's expectations to dash them all the more savagely. An utter travesty. Chaos Wars, which I never played, probably is the worst I ever heard, albeit second-hand via YouTube - http://www.youtube.com/watch?v=BAtC1SzWSXg