After the Sherlock Holmes crack I posted above, I'll be serious now.
Of course, William "Bill" Tilghman has got to be on everybody's list.
Another, on my list is Jeff Milton. He called Wes Hardin a liar to his face and lived to get an apology. By all accounts, Milton was a lawman of the finest character.
Third, although not a technically a lawman, Allen Pinkerton, founder of the famous Pinderton detective agency did much to bring many lawless types to justice.