Cal Dodd's Wolverine from the '90s show is still the definitive interpretation of the character in my mind. Of course he was the first one to get it right at all; both previous animated incarnations of the Canucklehead were inexplicably Australian. Being Canadian himself, Dodd naturally got the accent right. It took him a while to find the character; in the first season Wolverine sometimes sounded a lot like Popeye. But eventually he really mastered the character's savage rage, inner turmoil, and tragic depth. Subsequent Wolverine portrayers -- Hugh Jackman, Scott McNeil, Steve Blum -- just haven't managed to convey that same ferocity.
My other favorite from that show is Lenore Zann as Rogue. Sometimes she got kind of over-the-top, but most of the time her Rogue voice was very sexy. I loved the way it could be honey-sweet one moment and razor-sharp and shrill the next. That's range.
And George Buza did a fantastic job as Beast, really capturing his intelligence and humor. Although I don't quite consider him definitive anymore, since Kelsey Grammer did an equally good job in The Last Stand. These days, I sort of go back and forth between them in my mind when I read an X-Men comic.