What's the best filler for wrinkles?

That depends!
I use Juvederm more than any other filler. It is smooth, soft, natural looking and temporary. Juevederm will correct most lines - deep or shallow really well.
Juvederm does last only 6 to 8 months, though. That's OK for some, but some people want a longer duration in areas like the nasolabial folds. Then Radiesse is an option, assuming the folds are deep. Radiesse is not good for shallow folds.
I do not use Sculptra much for wrinkles. I'm not that impressed by the precision of Sculptra's filling. It is really great for filling in large areas like the cheeks, but not as good for well defined lines.
Some people really want permanent filling, in which case Artefill is a good option. I have injected a good amount of Artefill and the results are good. The technique is important, so if you want Artefill, you should go to someone who has lots of experience with it.
Other fillers are in the process of being FDA approved, but these are the best ones available today, in my opinion.