My list, as well, is rather extensive.
Real People:
George Bush
John Kerry
Paul Martin
Jean Chretien
Jim Henson
Robin Williams
Regis Philbin
Alex Trebek
Johnny Carson
Jay Leno
Penn (of Penn & Teller)
Amazing James Randi
Ed "Begley" Jr.
Brad Roberts (of the Crash Test Dummies)
Brian Wilson (of the Beach Boys)
Eugene Levy
Rowan Atkinson
Jeff Foxworthy
Ron White
Larry the Cable Guy
Jerry Seinfeld
Larry David
Jason Alexander
Larry Charles
Top 5 Muppets (these are in order):
1) Crazy Harry
2) Traveling Matt
3) Sprocket
4) Scooter
5) Janice
The Rest of the Muppets:
Pops
Dr. Bunsen
Gobo Fraggle
Baby Bear
Sweetums
Statler
Waldorf
Johnny Fiama
Angel Marie
Sal Manila
Piggy
Angus McGonagall
Ernie
Bert
Grover
Cookie Monster
Scooter
Fozzie Bear
Zoot
Animal
Floyd Pepper
Animated Characters:
Porky Pig
Daffy Duck
Sylvester D. Cat
Sylvester Jr.
Michigan J. Frog
Donald Duck (This is EXTREMELY hard to do, so I can only do it for 'short outbursts' as the case may be.)
The Tazmanian Devil
Scooby-Doo
Shaggy
Fred Flintstone
Fictional Characters:
Gandalf (Ian McKellen)
Indiana Jones
C-3PO
R2-D2
Jar Jar Binks (ashamed as I am of it, I can actually do his voice rather well)
Mitch (of 'A Mighty Wind')
Firmin & Andre; Phantom; Joseph Buquet; Madame Giry (of Phantom of the Opera)
Kramer; George Costanza; Lieutenant Bookman; J. Peterman; Marla; George's Mother; The immigrant with the pony (From Seinfeld)