is he the most relatable of the universal classic monsters?

I'd say Creature from the Black Lagoon is

I'd say The Wolf Man is the most relatable.

Yes, although for many, the Chaney, Jr. Wolf Man is up there, especially as he was, in non-wolf form, a man of conscience and moral integrity. In the Frankenstein monster there's an incipient humanity to him, yet he remains, due to the nature of his lab creation, essentially a dumb animal, albeit with some sympathetic human attributes.

I find the Karloff Mummy poignant, especially in his (admittedly) morbid romanticism, or obsessiveness, if one prefers a more clinical perspective. Dracula and other, similar vampire creatures, is nearly impossible to relate to, yet his aristocratic ancestry and, for a monster anyway, poise and good manners, make him stand out, and lend him a measure of charisma.

The Invisible Man, in the original 1933 film, is difficult for most of us to relate to due to his megalomania, sadism and overall narcissism, which came, for him, from the chemical that enabled him to become invisible, and was not inherent to his character prior to his becoming invisible. This, plus the fact that he can't be seen, make him wholly unlikable, unless, I suppose, the viewer has a yen for world domination and playing cruel tricks on innocent people.

And he wears a sports jacket. Real class that Frankie

Yes. He has issues of social acceptance like many people. Unattractive. Limited intellect therefore prone to being bullied. In some ways the 1931 movie was a teen movie well before the teen movies of the 1980's.