Tarantino has his favorite actors, like many other directors. Think Scorsese and Leonardo DiCaprio or Depp in Tim Burton's movies.
So, who is Tarantino's all-time favorite actor?
Tarantino's Style And Actors That Fit The Parts

Quentin Tarantino hasn't made a massive movie collection, but his name is attached to some of the most popular projects ever since From Dusk Till Dawn.
He has strong visuals, language, and some epic one-liners. Violence and blood are always expected, and you need a particular movie star to fit in with this director's vision.
Pulp Fiction, Reservoir Dogs, Kill Bill defined Tarantino's career, and we are reasonably confident that strong leads had plenty to do with the movies' successes.
Actors Who Are Tarantino's Favorites

Samuel L Jackson appeared in Pulp Fiction, Jackie Brown, Kill Bill Vol. 2, Django Unchained, The Hateful Eight. He also narrated part of Inglourious Basterds.
So, one would assume he is Tarantino's most used actor, which is partly true.
But the most used actor, who is not actually an actor, in Tarantino's movies in Tarantino himself.
He appeared in almost all of his ten movies, even if it was only a cameo. In that manner, Quentin Tarantino is similar to Woody Allen, who often guest stars in his films.
However, Tarantino's favorite is Jackson, followed by Harvey Keitel, Michael Madsen, and Uma Thurman when it comes to trained actors.
So to answer your question, which is Tarantino's favorite, that would be Samuel L Jackson. But, it appears that the director has to appear in his films, as this list proves:
Reservoir Dogs, Mr. Brown
Pulp Fiction, as Jimmie Dimmick
Jackie Brown, a voiceover
Kill Bill, a Crazy 88 member
Death Proof, Warren the Bartender
Inglourious Basterds cameos as a scalped Nazi and an American GI
Django Unchained cameos as Bag Head #1 and Frankie
The Hateful Eight narrator
Once Upon a Time in Hollywood, cameo