Hmm. Interesting question. A question with many possible answers, even from just my point of view. I think I'll break it down by category:
Films = Harry Potter - While not all of the films are as great as the others, they're all still fantastic. Quite a feat considering how many there are.
I almost put Lord of the Rings in here, but there've been some pretty awful adaptations over the years, especially in the animated department. So I just couldn't.
Games = Half-Life - I know there are other choices for this one, but this is one that just stands out to me. Not a single entry in the series has been half-assed (ba-dum tish) or phoned-in to bank on the popularity of the brand. Not to mention each entry is highly critically acclaimed. There really aren't many series in the gaming industry that can claim that. Even ones as "timeless" as Mario Bros.
Literature = The Lord of the Rings - I couldn't put it in the film section, but it deserves a space in this one. The Tolkien novels are masterpieces; ones untouched and unblemished by time or corporate greed. They remain timeless classics. Even if adaptations in other mediums have been...less than stellar.
Television = Star Trek - While not perfect the "batting average" of this series, in terms of characterization and quality writing, is WAY above the curve. It's still one of the most timeless and most influential sci-fi series ever created. With the advent of the new films and a recent increase in interest it looks to be coming back. Time will tell how it'll pan out.
EDIT
Comics = Calvin and Hobbes - Forgot to add this, primarily because the OP brought it up, but I felt I should maybe say why it should be added. Waterson so cherished his story of an obnoxious kid and his stuffed tiger that, even after he retired, he made sure his series would never be tarnished by bad adaptations or remakes. He turned down every offer he received to have Calvin and Hobbes reimagined by another cartoonist or to have it animated for a film or cartoon.
Hell, even those stickers you used to see on peoples cars weren't "official". They were spoofs or rips made by others simply to cash in.