I actually liked ol' Billy Bob as David Crockett. Funny, I was just thinking of Billy Bob as I read a letter of Crockett's from Christmas, 1834, lambasting Jackson and Martin Van Buren for their Indian Removal policies and threatening to go to Texas! I think Billy Bob captured the man who had been a great frontiersman in his younger days, but has gotten older, been a politician, and now found himself trapped in a hero's image he wasn't entirely comfortable with.
I'll say my favorite actor from a blackpowder era film is either Jeff Daniels in Gettysburg or Errol Flynn in Captain Blood. Now for Worst actor, well it has to be Orson Welles in the 1972 version of Treasure Island. I read and reread that book as a child, and Welles mumbled his way through the film, looking like he was just hoping they'd hand him the paycheck the producers were metaphorically waving in front of him, just off-camera.
Wait, I almost forgot - or more likely repressed - Kevin Costner in Prances... I mean, Dances with Wolves. I guess, however, that's really pushing it for "our" films, as it's right at the end of the Civil War and following, but it's still the percussion era in the West...