Welcome! Yours is a question that is asked often, but it is always interesting to hear what Charlie Chan fans have to say in answer.
As Chris stated, Chan fans are usually divided regarding their favorite actor to play the detective during the nearly two decade run of the series. Warner Oland and Sidney Toler usually run neck-and-neck with Roland Winters placing at third.
My personal favorite is Warner Oland. I like his gentle demenor, even in the most trying of circumstances. His Charlie Chan showed an obvious affection for each and every one of his many offspring, which I also find very touching. (Note in "Charlie Chan in London," when, as Mr. Chan begins to pack up his belongings for the trip back home to Honolulu near the beginning of the movie. When he reaches for the Chan family photograph, he smiles and takes a loving, almost cute, tickling "poke" at the young member of the family (at the end of the "family line-up.") I couldn never imagine Sidney Toler's Charlie Chan doing something like this.
Now, this said, I would like to add that Sidney Toler runs a very close second to Warner Oland in my book. The plus with Toler's Chan, I feel, is that his detective fit in very well with the times and society that was, by the late 1930s, in the process of a very rapid transformation (much as our society changed during the '60s). I have wondered how Oland's Chan would have fared. Charlie Chan, as portrayed by Sidney Toler had something of an "edge" that was well-suited to what was coming. (As one character, Jeanne Bentley, states in "Charlie Chan in Reno," "This is 1939, we're MODERN!")
Roland Winters was at a very real disadvantage. First, he had two VERY big pairs of shoes to fill! To follow both Oland and Toler must have been daunting. I feel that he did a very good job, and his version of Charlie Chan took the character on yet another path. In some ways, Roland Winters' Chan literally took a page or two from Earl Derr Biggers' characterization of the detective. Had he had more than the six opportunities to play Charlie Chan that fate had offered him, I think that he would have been remembered even more fondly for his portrayal. As it was, very late in his career, many people still remembered him mostly for his Charlie Chan movies!
I hope that this has provided something of an answer to your question. As mentioned, this question is asked with some regularity, but it never ceases to provide a new insight or two in this "great debate."
