Many a satirist has foundered on the rocks of George W. Bush - a monumental and surprisingly difficult subject. It’s the proverbial cow’s arse with a banjo, the mosquito in a nudist colony - when the bullseye is a mile wide how does the master satirist sharpen and differentiate their work from the amateurs?
Can be a comedian, cartoonist, writer - anyone. Brute force or deliciously nuanced - who skewered the man most effectively?
The Onion.
I think Frank Caliendo is pretty good at lampooning his behavior, especially his mis-statements. Steve Bridges is a great impersonator, though. He has GWB’s mannerisms and voice almost perfect.
Jon Stewart.
I do love that little troll laugh he does when making fun of Bush.
James Adomian. (YouTube link)
His impressions bring another level of character to whoever he’s doing. His Jesse Ventura is great too.
Will Farrell. It’s the part he was born to play, baby!
Will Farrell, hands down. There’s simply no question here.
Yeah, Will Farrell owned it. Caliendo is just a clownish imitation.
Will Ferrell??!!? You’re kidding me!
Jon Stewart’s brilliant but he built his latter day career on skewering the Bush administration and it’s policies, as well as the news media, as a whole. (With the help of the staff of the Daily Show.)
Are we talking about a specific impressions? Because Jon’s not much of an impressionist, as I’m sure he himself would admit. He doesn’t really do voices or characters.
ETA: The troll laugh really is funny though. 
He didn’t only satirize Bush, but I don’t think anyone did it better. To be honest I can’t tell if the OP is asking about people who made fun of Bush or people who did impressions of Bush. If he means the first one, Stewart. If it’s the second, someone else- although I think that malicious little laugh summed up the Bush era very well.
If were talking about who best skewered the Bush administration, I think Stephen Colbert’s performance at the WH Correspondents’ Dinner is the winner.
Caliendo’s voice and delivery was perfect, but Will Ferrell’s material was funnier.