By any standard, the movie adaptation of the board game Clue is not a big comedy. It feels like a small, amusing play that may not be worth the full $20 movie admission (hopefully it was cheaper back then in the 80s), but as a late night movie you catch on syndication TV or streaming on Netflix (as I did) you don't feel like you wasted your time.
The cast is pretty good (with one notable exception I'll talk about later), and the pacing is fast enough that would won't notice that many of the jokes kind of fall flat. Co-written by John Landis and Jonathan Lynn (also the director), you can feel the mixture of mid-80s American comedies and British stage-like comedy.
The multiple endings is not much of a spoiler now, since as a theatrical run the "ending number" was placed next to the billboards to entice viewers to see all 3. I'm still curious about that cut 4th ending that was especially dark. Now at home you'll see all 3 and be able to compare which one to prefer (3, 1 and 2, from best to worse for me), and I can imagine that without those few added minutes the movie in theatre would have felt quite short at barely 80 minutes.
So, for an average, enjoyable comedy, why is it now considered a classic? Well, simply because this may have been the best role that Adam Curry performed. While the many British actors that were originally cast for the role of the butler would have played their usual over-the-top (Rowan Atkinson and John Cleese), Adam maintains a perfect line between passion and insanity. In a next-to-final scene, he retells the entire movie in a hurry of just a few minutes, as if spoken in a single breath while running. Regardless of all the other cast, he effectively (and not only with that scene) elevates and pulls the entire movie by himself.
So, go watch it if you've never seen it. You'll will laugh, and is an almost guarantee that it will be better adaptation and more enjoyable then the soon-to-be-released other board game adaptation Battleship.