What is best to correct forehead dent/irregularity? (Photos)

Before jumping to treatment options you need to get a clear understanding of what is causing the contour issues.
It looks like you have a bossing of your brow. The subcutaneous fat layer has become thinner with age and you can see the outline of your corrugator in frontalis muscle. on some people the front talus muscle is not present in the midline and it creates two arches. The medial part that has no muscle covering usually starts higher in the forehead but in your case I believe it begins much lower down. If the problem is not related to bone contour in regards to indentation then methacrylate is probably not the correct treatment.
That treatment is very good for a deficiency of bone like an in fractured frontal sinus from many years ago.
Reducing the bossing of your superior orbital rim may be one option. This is not easy or straightforward and different approaches may be needed.
Anything that’s complex definitely warrants in person consultations with local board-certified plastic surgeons.
Fillers are far superior to fat transfer in regards to being predictable precise and forgiving.
I do not recommend you attempt fat transfer as your first attempt. Make sure the provider can give you the contour you’re looking for it with fillers first. The problem is that the higher areas have more post tissue and will retain fat much better in the shallow areas where you really need the volume and are devoid of host tissue. Fat graphs do not survive well if there isn’t good host tissue to support them bone will not support grafted fat. Your natural subcutaneous fat layer is very thin in this area and much of the contour issue is related to the edges of your muscle. You need the volume where the muscles are not present because that is part of what’s causing the indentation. The fuller areas where muscle is present will allow fat to survive much better potentially making the situation not well suitable to fat transfer.
If you find someone who tries using filler and you get good results from this consistently then down the road you could consider trying to convert to grafting fat. Reducing the supraorbital ridge and the use of fillers in combination may be helpful. Have you tried using Botox? weakening your Frontalis muscle but especially your corrugator muscles with the eventual atrophy that comes with it may be helpful.
This really requires an examination and potentially you should consider getting a three dimensional CT image created. A lot of oral surgeons have sophisticated facial CT machines they can generate good three-dimensional images of bone structure that are sometimes more financially friendly then using medical centers which sometimes bills egregious amounts of money regardless of what they collect.
I suggest having multiple in person consultations with local board-certified plastic surgeons to see what they have to say.
Best,
Mats Hagstrom MD