I have to agree with previous posts stating that the Montreal Screwjob, the past between the Undertaker and Kane, or really any Undertaker type of story would make for the best films.
I also agree with the Kane/Cena "Embrace the Hate" storyline making for a good film. To this day I can't understand why so much of the IWC crapped on the storyline, as up until Eve got involved, that was one of the best and most interesting storylines WWE had come up with in awhile. It was very much like the Dark Knight, and it deserved far more praise then it got.
I'm gonna throw one in there that hasn't been mentioned and that's the saga between Macho Man Randy Savage and Miss Elizabeth. It's the best "love story" ever portrayed in the WWE that lasted for almost a decade. Randy Savage comes in as this hot-shot, gifted wrestler who takes his woman for granted and treats her like crap. He sees the error of his ways and treats her better and reaches the pinnacle of the business becoming champion and becoming allies and friends with the most powerful wrestler in the world (Hulk Hogan). Only for his paranoia, insecurity, and jealousy to ultimately rip his friendship with Hulk Hogan apart, and more importantly rip his relationship with Elizabeth apart. Then Savage finds a new woman (Sensational Sherri) and becomes a mean, vengeful, dangerous wrestler, who just isn't as happy and successful as he once was (which was true as he was mostly in mid-card feuds for the remainder of his heel run). His downfall concludes with him losing his career and livlihood to the Ultimate Warrior at Wrestlemania VII. Then his new woman betrays him and shuns him for losing and kicking him while he's down in the ultimate humiliation. Then the love of his life who's there live, can't stand to watch, comes in and saves him by beating up the new woman. Savage awakens and realizes that Elizabeth has saved him and realizes that she's always loved him, and in the process he never stopped loving her. They embrace in an emotional, heartfelt moment before 20,000 screaming fans, and then are married months later in a fairtytale happy ending (Summerslam 91). It's the perfect movie.
Of course you could add more to it with deranged, psychotic heel trying to hurt both of them (the feud with Jake Roberts and Undertaker), and then of course the playboy, ladies man trying to tear the two apart (the feud with Ric Flair), but I think the wedding is a good end as it would make a happy ending, which is what you need for a Hollywood film.
Another good one would be the story of Shawn Michaels and Undertaker. Two iconic wrestlers of a generation whose paths rarely crossed, until the end of their careers in which the Undertaker has to send Shawn Michaels to his retirement. That would be a good movie as well.