I don't know if he was the best, but Basil Rathbone is always mentioned as an excellent fencer. He was much more proficient than Flynn, but being the villian in both Captain Blood and Robin Hood, he was destined to lose.
Yup Basil was the man. He also was quite good as the Captain in Zorro.

Heres a guy that you wouldnt think of as a swashbuckler but did it quite well. Gene Kelly in the Three Musketeers. I hated when he spoke but the fighting scenes are quite spectacular. I was amazed to see how mant versions there are of this movie. Heres a few of the more interesting ones.

The Gay Musketeer (1928)
I don't know if this is in any way related to the story of D'Artagnan, Athos, Porthos and Aramis, but would point out that this movie predates (I believe) the modern usage of the word "gay".

The Three Musketeers 1933
Directed by Colbert Clark; starring John Wayne, Raymond Hatton, Lon Chaney Jr., Francis X. Bushman, William Desmond, John Qualen and Noah Beery Jr. A serial of twelve chapters totaling 215 minutes, in which the story is translated to North Africa, with the Foreign Legion taking the place of the King's Musketeers.

The Three Musketeers 1939
AKA The Singing Musketeer. Directed by Alan Dwan; adapted by William A. Drake, Sam Hellman, Ray Golden, Sam Hellman, Sid Kuller and M.M. Musselman; starring Don Ameche as D'Artagnan and the Ritz Brothers comedy team as his [sorta] brothers in arms. 73 minute movie musical comedy version of the classic story.


The Three Musketeers 1948.
Directed by George Sidney; adapted by Robert Ardrey; starring Gene Kelly, Van Heflin, Gig Young, Vincent Price, Lana Turner, June Allyson, Angela Lansbury, Keenan Wynn and Robert Coote. This 125-minute screenplay manages to cram an amazing percentage of the story into a single standard-length movie. The humor and Gene Kelly's D'Artagnan are somewhat broad, but the movie is very good and the scenery and costumes make full use of the Technicolor process. I particularly like Van Heflin's Athos.

Heres a link to more versions
LINK