Help Channel's Voice Actor list.
Honestly I don't think I've ever tried to pair the characters up with a voice actor. I mean certain characters of course sound a certain way to me (in my head) when reading dialog and such, but they're never the voices of Hollywood types, not that I'm consciously aware of anyhow.
There's some, interesting pairings in there.

@Oathbound
@Oathbound Too

I think Bruce as States would be good casting. He could actually make me give a damn about that character.
One that came to mind for me would be Robert Picardo as Positron/Anti-Matter.
I approve of Terry Crews voicing BABs.
This is just wrong on so many leve...fun choice.

An interesting choice. He should be able to pull it off. I might have said Danny DeVito, but I've never heard him shake his Jersey accent. Scorpion is South American, and I see him as something of a banana republic tin-plated despot. Maybe Cheech Marin? I've seen some of his more serious stuff. He should be able to strike a balance between psychotic Scorpion and humorously psychotic Scorpion.
Nemesis is Prussian. So Stewart shouldn't...hmm...French captain with English accent...nevermind, carry on.

I tend to hear Arleen Sorkin's voice in this role.
I always imagined actor/singer James Darren (Vic on Deep Space 9) in the role. Sonata even looks a bit like Darren, in a roughed up, hard life kind of way.
Strictly for the emphasis on the golden voice, though it sounds a bit young, Michael Bublé might work as well. I don't know his acting skills, however.
I approve of A LOT of your choices up there. Many of them make perfect sense.
I know it is a bit stereotypical, but I would totally cast Christopher Lee as Rularuu the Ravager.
Some others I'll just throw out there, since we are talking voice only:
Sunstorm: Liam Neison
Ms. Liberty: Emma Stone
Doc Delilah: Michelle Pfiefer
Faultline/Jim Temblor: Jeremy Renner
Fusionette: Abigail Breslin
@Winter. Because I'm Winter. Period.

I am a blaster first, and an alt-oholic second.

Hmm, if it's a matter of voices, I'd have to say the following:
Ghost Widow: Jennifer Hale
War Witch: Tara Strong
DJ Zero: Billy West
Back Alley Brawler: John DiMaggio
Romulus: Patrick Warburton
Statesman/other versions - Bruce Willis.
Manticore/Chimera - Nicholas Cage.
Back Alley Brawler/Marauder - Terry Crews.
Infernal/Infernal - Keith David.
Valkyrie/Battle Maiden - Uma Therman.
Citadel/Luminary/Seige/Nightstar - Stephen Hawking.
Synapse/Nueron - Ryan Reynolds.
Faathim the Kind - Leonard Nemoy.
Lord Recluse - Patrick Bateman.
Kalinda - Terri Brosius.
Arbiter Sands - Keanu Reeves.
Capn' Mako - Hugo Weaving.
Black Scorpion - Jack Black.
Wretch - Vin Deisel.
Ghost Widow - Rebecca Romijn-Stamos.
Agent Indigo - Milla Jevovich.
Agent Crimson - Sam Jackson.
Silver Mantis - Lucy Lui.
Snaptooth - Dane Cook (for reasons.)
Dr. Vazhilok - Ralph Fiennes.
Lilitu - Barbara Goodson.
Countess Crey - Michelle Pfieffer.
Rularuu the Ravager - Chuck Norris.
Romulus - Gerard Butler.
Malaise - Johnny Depp.
Lord Nemesis - Sir Patrick Stewart.
Lanaru the Mad - Sean Bean.
Mother Mayhem - Lynda Carter.
Mr. Yin - Jackie Chan.
Penelope Yin - Lucy Lui.
Flambeaux - Megan Fox.
Johnny Sonota - Richard Cheese.
It's all I have written down from the extensive help chat.