Carl Barks created the Duckverse. Don Rosa created "The Life and Time of Uncle Scrooge". Generally, all comics by those two artists are considered canon.
Apart from those two artists, which other artists (or stories) are considered Duck canon?
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It depends highly on what you consider canon. There are numerous Donald Duck stories published in Europe, mostly by Italian and Spanish artists, now under the auspices of Danish publishing house Egmont. Being official, licensed productions, they can be considered canon.
This site has a list of all Disney comic book artists, but doesn't distinguish between Donald Duck, Mickey Mouse, and other characters.
A lot of characters that are now considered canon were created by artists other than Barks and Rosa. Dick Kinney and Al Hubbard created Donald's cousin Fethry Duck, while Moby Duck was created by Vic Lockman and Tony Strobl (who drew quite a lot of US Donald Duck stories.
Al Taliaferro, with writer Bob Karp, did the Donald Duck newspaper strip from 1938 to 1968 and Sunday pages from 1939 to 1969. The strips introduced in comic form (both had appeared in cartoons) Donald's cousin Gus and love interest Daisy Duck, as well as Grandma Duck, all of which must be considered canon. Taliaferro also created Donald's iconic car. Much of this happened before Carl Barks began on the comic book in 1943, and Barks used many of Taliaferro's characters. If you consider Barks canon, you hence must also consider Taliaferro canon.
A popular superheroic version of Donald Duck, Paperinik or Duck Avenger, was created by Italians Guido Martina and Giovan Battista Carpi. One of the most productive Duck-artist was Victor "Vicar" Arriagada Rios, (deceased 2012). He had his own studio where he and his assistants drew the stories sent in by Egmont.
Some fans don't consider any post-Barks stories canon except those by Rosa (and some even don't count these), while others consider all licensed stories canon. It depends on a matter of taste; I doubt that Disney has any official opinion on the matter.
Wikipedia has more detail about the development of Donald Duck comics.