As well as Grey Delisle (not only as Frankie but as Mandy), here are my favourites:
Tara Strong - A very talanted voice actor, who has voiced great characters for great cartoons (including Ben 10, Teen Titans and Beetlejuice). My favourite from her is Raven from Teen Titans. She really brought out the creepyness (no offence to Raven
) and even sometimes made it funny without becoming funny. An example of that is the first episode when Starfire offers her Pudding of Saddness to Raven, to which she replies: 'My mind is never troubled. People come, people go. It's pointless to be upset about Cyborg.' The computer behind her breaks, Starfire looks shocked and Raven just goes...
'What?'
Seth MacFarlane - Another talanted guy who really shows it on his show Family Guy. He makes his charaters that he does really funny, and shows that it's sometimes the way that you say something that makes it funny.
I'm also a fan of his sister, Rachel MacFarlane. Being a fan of the 1/362 pairing on Codename: Kids Next Door and having seen Operation IT, she made the character of Rachel (aka Numbuh 362) what she is today.