If you have never used a facial filler before, consider one of the more temporary fillers, so you won't have to live with a (potentially bad) result you didn't anticipate for a long time. Perlane (and Restylane or Juvederm) can last 6-9 months or longer, and will take a minimum of two syringes for visible changes (1 per side). This will help you figure costs. Radiesse (same 1-2 syringes) can last for a year or perhaps more, and Sculptra has a slightly longer persistence (18-24 months). Artefill is touted as "permanent" but only a small component of the total injection volume persists, also giving this filler the reputation as temporary. You should know that one-third to almost one-half of patients find Artefill, Sculptra. and Radiesse to be "not worth it." I would suggest that this is generally due not to the fault of the product, but to the skill of the injector in conjunction with the promises made by the injector to the patient and the cost charged. Consider other answers here as to which products are better for dermis, soft tissues, or deeper contour, as each has its pros and cons. I certainly would be upset to pay a lot for "smooth and long-term" if I ended up with "lumpy and way-too-long-now-that-I-hate-it!" Conservative and a bit at a time is wise for any patient (and doctor).
If you have had facial fillers before, like the effect, and wonder what lasts the longest, see above, but consider that surgical implants (properly placed by experienced, board-certified plastic surgeon or facial plastic surgeon) are indeed permanent, and may save you money in the long run. They can be removed if this proves to be a mistake, but for the right patient this is really the way to go for longest-term and most predictable smooth result.