What is the best way to treat my vertical forehead dent? (Photos)

Thank you for the photographs, although in-person evaluation is essential.
It is possible that the dent is due to strong Glabellar-complex muscles, which can be treated with neurotoxins such as Botox/Dysport and Xeomin. 
This part of the face is considered a "dangerous zone" (although there are no "safe zones" in the entire face) due to anatomical location of the supratrochlear blood vessels. An accidental injection of the filler can cause compications like blindness. 
However, sometimes even prominent veins can cause such a depression on the forehead. 
Get a consultation with an expert aesthetic physician and discuss all options.
Good luck and best wishes.
Dr. Shanthala

The photos suggest that the "dent" may actually be resulting from tension within the expression muscles of the glabella to the sides of the depression. If this is indeed so, then the use of a neuromodulator, such as Botox, Dysport or Xeomin, would help to reduce the tension and lower the "mounds" to the sides of the dent, in effect smoothing the entire region. If filler is still needed to achieve further smoothing, then a soft, cohesive filler, such as Belotero Balance, which can be injected quite superficially without worry of causing the unwanted, bluish Tyndall Effect when injected higher up in the skin, may further improve the overall appearance. While your concerns about problems from injecting fillers in this region are legitimate, in experienced hands, these are quite rare and actually have been more associated with the use of thick volumizing agents, such as fat injections in this location. Make sure you consult with a board certified aesthetic physician with experience and expertise in nonsurgical facial rejuvenation using injectable fillers and volumizers. Good luck to you. 

This indentation is something that comes from too many frowning motions.  After a period of time it needs to have both filler and Botox to get back to where it was.  I generally use Restylane for this area but it is a tricky area so you need to make sure that you are with someone who knows what they are doing. Good luck!