I've worked in the film business for several years, and while I wouldn't say his career bombed, I think the most disappointing acting career belongs to Roddy Piper.
Before people get upset, let me explain
There is one huge movie star in the history of wrestling, Dwayne Johnson (who is arguably one of the 10 biggest movie stars in the world considering the global appeal of his films).
Hulk Hogan's movies and tv shows may have been awful, but I think he was always viewed as an attraction rather than an actor. And he made a lot of money making a lot of B movies, so he really met expectations.
Andre the Giant could never be a movie star with his size
unfortunately, it always limited the available roles. But he has a hit with Conan, and gave arguably the best performance of any wrestler ever in The Princess Bride.
Steve Austin, like Hogan, has made a number of B movies (plus small parts in bigger Hollywood movies), and probably has done as well as he could expect.
It's too soon to judge guys like Cena and Batista, although I suspect they'll be limited to B movies.
And now to the Hot Rod
one of my favorite wrestlers of all time. And a hell of an entertaining action star in They Live. He was a pretty big dude, imposing enough to be a believable action star, without being too freakish like Hogan and Andre. He could be funny as well as intimidating. And he had a natural demeanor on camera. Hogan was super charismatic, but could not deliver natural dialogue. With all due respect to Dwayne Johnson, I think Roddy was the most talented actor in the history of wrestling. Unfortunately, I suspect timing killed his career. He tried to break into Hollywood at a time when the studios weren't willing to consider wrestlers for big films (unless they were playing wrestlers, and No Holds Barred was not that big of a film, at least budget wise).
Because he had the talent, but was unable to fulfill the promise of that talent, I rank Piper's acting career as the most disappointing (even though it was arguably as commercially successful as anyone other than the Rock). Piper was capable of being a huge star given the opportunity and right material. Things just didn't shake out that way.