"This one . . . well, I think this one is really gonna be a neat movie," says William Shatner of his next film, in which he once again returns to pilot the Starship Enterprise through space, the final frontier. Enjoying his first day off after wrapping up the season's episodes of "T.J. Hooker," Shatner had nothing but wide-eyed praise for the second big screen Star Trek, which has been given the title "Star Trek II: The Vengeance of Khan." "I've had more fun, and more enjoyment, working on this movie than anything I can remember in a long time," says the man who will forever be known as Captain (or, in this film, Admiral) Kirk. "And that's really why I do this." Normally cautious insiders at Paramount Studios also are beginning to speak enthusiastically of the film, directed not by a known science-fiction filmmaker but by Nicholas Meyer, author of "The Seven Percent Solution" and director of "Time After Time." And these advance raves are more than either Paramount or Shatner were willing to give the original "Star Trek: The Motion Picture" when it came out a few years back; "it was flawed," concedes Shatner readily. "It was a flawed movie that was intermittently entertaining, and I think you probably got your money's worth. But this next one . . ." This next one, by the way, is the film in which Ricardo Montalban returns as Khan, a role he originated in an old television episode of the show, in which Kirk's son plays a crucial role, and in which Leonard Nimoy's Mr. Spock is killed, probably. Paramount's official word on Spock's demise has changed over the past couple of months: Where they used to note, "Mr. Spock's health is of utmost importance to all of us at Paramount," lately they've been retreating behind the equally hazy "Nobody ever really dies in science fiction."