For me it was Jeremy Brett.
The series he did for Granada Television (shown in the States on the PBS Series Mystery!) was really the first time both Sherlock Holmes and Dr. John Watson were portrayed the way Sir Arthur Conan Doyle had written them. Watson especially who had, with the exception of a handful of films, had always been depicted as a blithering old idiot.
This all changed when David Burke took the role of Watson. Most every production that followed, have depicted Watson very much the way the Granada series did.
In many of those early episodes, I feel as though Jeremy Brett and David Burke resembled Sidney Paget's artwork so much, you would almost think they walked right out of a copy of the Strand Magazine.
Although, in real life, Jeremy Brett actually wrote Lady Jean Doyle (the daughter of the late Sir Arthur) and asked her permission to have Sherlock Holmes kick his addiction to cocaine when he found out children in England looked up to the character and considered him to be a superhero of sorts.
The character is seen emptying a phial of cocaine and burying his needle on the beach in the episode The Devil's Foot.
This was also the first time I feel we also saw in my opinion the definitive portrayal of Holmes's arch-nemesis Professor James Moriarty in the form of Eric Porter.
Although the character remains in check, you do get the feeling that he could become unhinged at any moment with this face twitch Eric Porter would use as the character.
I've gone on the record (and got my fair share of grief) for saying I am not a fan of Andrew Scott's "Jim" Mortiarty from the Benedict Cumberbatch series. Which isn't a shot at the actor himself. I haven't seen enough of him to draw a conclusion of his skills as an actor. But I just don't like the whole "goofball" element they added to him.
To be honest, I liked Jared Harris's take on Moriarty in the Robert Downey Jr. film better. However, I like the Benedict Cumberbatch take on Holmes better.
With that said, I'm not going to ridicule you just because I didn't care for the Moriarty from Cumberbatch series. Or leave nasty notes about it on Steven Moffat's Twitter account telling him how much I dislike the way he has interpreted the character.
That would be stupid.
If you're new to Sherlock Holmes and have become interested in the character because of the Benedict Cumberbatch series, I would highly recommend you check out the series starring Jeremy Brett. Although, it may not be as high-octane as some of the current interpretations of the character, it is highly enjoyable.