PRP + ACell, Amniotic stem cell, or Exosomes? Which one of these treatments is most effective for hair loss and why?

In  my opinion, no Validated Studies have been performed showing that stem cells, Acell or exosomes add any additional value to the PRP alone. In fact, A cell is a foreign protein, it can cause additional allergic reactions and side effects. Using these extraneous agents also requires additional injections and puts pressure on the hair follicles. This can lead to an increase incidence of shock loss. Topical exosomes provide no additional benefit over topical PRP. Find a PRP expert. Ask to see photos.

Great question. However, you can fill a few books covering the subjects in detail. A few highlights are:
Exosomes: I'll address this first because it is the easiest to address. Exosomes represent a very promising field in medicine! In fact, I'm all over it trying to learn as much as I can on this subject.  There are no FDA approved products on the market that can be injected. 
I know there are some topical cosmetics claiming to have or are based on exosomes. However, each time I try to evaluate these products I'm left with many reasons why I'm not quite sure of what to make of them or their claims.
I do have a few friends that tried exosomes derived from Mesenchymal Stem Cells in a liposome cream. This delivered bad to very good results. The bad results were too common to consider bringing a product forward.
Bottom line: Avoid the hype for now. Too many unknowns in the cosmetic industry. There's little telling what they are doing in terms of formulation to be able to claim true unadulterated exosomes in their products. Certainly, none of it can legally be injected. Using it as a topical is of serious dubious beneficial benefit. Nor would I microneedle any of the products.
I expect one day in the not too distant future exosomes will provide a significant breakthrough. 
Stem Cells: There are darn few stem cell products approved by the FDA to date. None of which are suitable for hair regeneration. 
Adipose stem cells harvest from your fat and bone marrow stem cells have proven to be less than satisfactory for hair regeneration, but great for other things.
Masquerading as "Stem Cell Therapy" by many physicians are indeed a number of amniotic allografts that indeed contain stem cells. However they are non viable stem cells. Therefore, there is nothing in these products that have much practical connection to having "Stemness". 
Yes, it is widely believed that a few percent of the stem cells in some of these products are viable. However, there are not enough to be able to make any big scientifically valid claims about the gross benefit of so few stem cells.
What they do have that is undisputed, is they possess a very rich and potent amount of all the growth factors associated with a fetus. This indeed provides fertile grounds for trying them in hair regeneration. In my opinion, the cost of these products out weighs the benefit. Products from Invitrx and Liveyon are probably the best to use.
Extracellular  Matrix (ECM): There are a number of ECM's on the market. A few types are, by order of cost and efficacy (less to highest):
Porcine bladder matrix (Acell), amniotic (many companies). Assuming you do not have an allergy to pork, this is a good product.   
Amniotic (many companies). I've tried a number of these, I'm not so sure the extra cost provides bang for the buck.  
Placental matrix (a few companies)
Considerations: 
If trying to regain hair loss from 3 to 5 years ago, definitely include ECM in your treatment plan.
If over 50 years old, you should seriously consider adding ECM.
If over 70 definitely include ECM.
If money is not an object, go with the placental matrix. The good one's are nearly as much in cost as the allografts and seem equal to them in efficacy. 
If money is a consideration, go with the Acell, it has a good efficacy and for the money, nearly as good as the amniotic. 
Hope I have helped. Wishing you a wonderful hair regeneration outcome.
Warmly,
Arbella Sarkis, MD

Exosomes, may one day, be the gold standard but at present they cannot be injected and will add a lot of cost. I would certainly start with PRP, since it is very easy have done to you, and have virtually no side effects. you need to plan to do 3 sessions, each spaced a month apart.david berman md