It depends on where the fillers will be placed. There are a number of ways to alter the lip contour and the visual appearance of fullness.
For injecting in the actual body of the lip, Juvederm and Belotero are great soft gel products.
For injecting around or "behind" the lip - when actually injecting "into" the lip is not required or not desired, you can achieve some natural lip eversion (turns out) with either of the fillers mentioned or Restylane or even Perlane.
The entire lip area should be viewed as a unit - sometimes there needs to be some additional substructure support above, below, behind and adjacent to the lips themselves to keep everything in a natural-looking proportional balance.