Assuming one wants no "enhanced" color to the slide, Kodak's EPN slide film is the most neutral film I found, however, I have not shot art work for a couple of years, so I've no experience with any of the newer films. Most new films seem to promote themselves as some version of a saturated or color shifted film which in most cases would not be approriate for accurate color reproduction. Color bars are only going to give the printer a reference point. The printer will generally not be able correct any oversaturation issues, even with color bars.