I have both audio by all three, since I work the night shift I have plenty of time to listen to them.
My first audio book was by Tony and was Wee Free Men and I thought it was great! Then I got Brigg's and that was fantastic.
I have a lot of Planners and I think for the Witches books his is the best. His Nanny Ogg just gets me rolling.
I like Briggs for the Watch books, but not for his Colon (TEEHEE!

) which I think Planner does better.
For Death books both Planner and Briggs does a decent voice of Death, but Planner has done the bulk of the work so I think he does better on that.
Briggs range of voices for all the different races though just blows my mind! His work in 5th Elephant shows it all. Trolls, Vampires Werewolves, Igors, Nobby is a work of brilliance and also in The Truth the swithng back and forth of voices amazes me. So it is hard to choose just one for me. So I choose A and C.
As a side note, can you please add the names to the Polls so it is easier to vote. Kind of hard after typing all this to remember who is who.
Also, I need to add that I like Planners Vimes, even though Vimes was sober in the later in the books Vimes is sober Planner keeps his 'roughen drunkeness' quality to his voice. But Briggs has added the toughness that a commander needs in his voice.
Funny enough both Planner and Briggs Carrot and Nobby is just about the same, and either of the Vetinari is spectaular in their own way.