There are a few factors to consider when determining what makes a good fly spray.
1) The fly spray must do an exceptional job of repelling mosquitoes, ticks, and nuisance insects. Citronella (Mahmud), peppermint (Ansari et al. 2000), geraniol (Mullens et al. 2017), and neem (Garboui et al. 2006) have scientific studies demonstrating their effectiveness. An effective fly spray can use plant-based compounds individually or blended. Blending essential oils can create a final mixture that provides the benefits of multiple individual oils.
2) Another consideration is how long the fly spray will last. Mullins et al. recommends reapplying geraniol at 1-2 day intervals. However, if it is convenient herbal fly spray can be reapplied more frequently.
3) Safety is important. Herbal fly sprays provide an alternative to chemical fly sprays that is safer to the environment (Mahmud et al. 2022).
4) Personal preference of the smell of a fly spray can be important but is often overlooked. The addition of peppermint can dramatically improve the smell of a fly spray in addition to improving the fly repelling qualities. Remember, you will be smelling your fly spray for as long as you use it.
5) A critical but often overlooked consideration when determining what makes a good fly spray is the sprayer system. Even the best fly spray will not work if the sprayer fails to apply an adequate and even layer of fly spray. Sprayers for barn use need to be reliable.
6) Finally, you should consider the feel of the spray. This is not intuitive, but different types of fly sprays can feel very different. Some sprays coat with a thick oily residue. This can often attract dust and make a horse look dirty. You may not think you will touch a fly spray, but you likely will if it is on your pet's coat and you groom or pet them. You will also touch a fly spray if it is on your cloths. Depending on the way you plan on using your fly spray feel may or may not be a consideration.
Ansari, M. A., et al. "Larvicidal and mosquito repellent action of peppermint (Mentha piperita) oil." Bioresource technology 71.3 (2000): 267-271.
Garboui, Samira S., Thomas GT Jaenson, and Katinka Pålsson. "Repellency of MyggA® Natural spray (para-menthane-3, 8-diol) and RB86 (neem oil) against the tick Ixodes ricinus (Acari: Ixodidae) in the field in east-central Sweden." Experimental & applied acarology 40.3 (2006): 271-277.
Mahmud, Faiza, et al. "Biological & insecticidal effect of citronella oil: A short review." Clinical Medicine And Health Research Journal 2.6 (2022): 261-265.
Mullens, Bradley A., et al. "Field trials of fatty acids and geraniol applied to cattle for suppression of horn flies, Haematobia irritans (Diptera: Muscidae), with observations on fly defensive behaviors." Veterinary Parasitology 245 (2017): 14-28.