Jerry Nelson, Steve Whitmire, and Dave Goelz.
And since I feel like explaining:
I love Jerry Nelson's voice. He has such a wide range of characters and they all have their own personalities or quirks (Floyd's walk, Gobo's accent, etc). His voice is so versatile and his range is huge. And he is a fantastic singer.
Steve Whitmire is also a fantastic singer: he can scat like NOBODY's business!! His characters are fun and lively, and believable, and they're all different.
Dave Goelz is awesome, because he always makes Gonzo seem just so alive, with so much heart. Gonzo seems to me probbaly the most human of any of the muppets. And I like hearing him sing as Gonzo or Travelling Matt, too. ^_^ Very unique voice, it's got a homey quality to it.