Many of those mentioned aren't really voice actors but actors who have lended their voices to characters. Liam Neeson for example, excellent voice but he just used his normal voice. A voice actor is somone who can act with their voice not just speak... am I being clear or am I rambling for no reason?
Okay, the voice actors and for what voices I chose them.
Billy West, (Fry, Farnsworth, Zoidberg inter alia, Ren, Stimpy, Doug, many of the Loony Tunes.)
Rob Paulsen, (Pinky, Yakko.)
Maurice LaMarche, (Orson Welles, The Brain, a hundred Futurama characters including the Scary Door guy.)
Richard Steven Horvitz, (Zim, Raz from Psychonauts, many Nickelodeon and CN characters.)
Dan Castellaneta, (Do I even need to say?)
Patrick Warburton, (I know he pretty much only does the one voice but I think it's cool.)
Seth MacFarlane, (Again, don't need to say. Don't like Family Guy or his other shows but he's a decent voice actor for certain.)
Aaand many more, (David Herman for example,) but I'm not bothered to list 'em.
Not mentioning Mel Blanc because I shouldn't have to mention Mel Blanc. The guy is voice acting God, he has no equal.