The New York Post said the letter was posted on Facebook by Steven Michael Quezada, Cranston's co-star on the show, which wrapped last month.
"I have never watched anything like it. Brilliant!" Hopkins told Cranston in the letter.
"Your performance as Walter White was the best acting I have seen -- ever... this work of yours is spectacular -- absolutely stunning. From what started as a black comedy, descended into a labyrinth of blood, destruction and hell. It was like a great Jacobean, Shakesperian or Greek Tragedy."
Hopkins also asked Cranston to pass on his "admiration" to his co-stars.
"That kind of work/artistry is rare, and when, once in a while, it occurs, as in this epic work, it restores confidence," Hopkins wrote. "You and all the cast are the best actors I've ever seen."