There is no perfectly "neutral" film. Any good studio always tests film for a given setup and the current film emulsion and lab combination. Stay away from the overly saturated films, such as Fuji Velvia and the Kodak "high color" films, and shoot with the Q-14 or Gretag-Macbeth color checker (not the digital version), and stay away from "portrait" films that tend to mute colors or emphasize skin tones. Run one sheet of film or a "clip test" or same-emulsion roll of film, then check for color balance and exposure and adjust with color correction filters on your final shoot. There's a small (a few inches on a side) version of the Gretag-Macbeth color checker for inclusion in the scene when doing art copy work. It costs about the same as the larger version. I haven't shot a lot of the current color transparency films, so I don't have specific suggestions there.