Background: I once worked for an ad agency that had among its clients Vidal Sassoon, and from them I learned all the secrets of the trade. Below is just a sample.
At its most basic level, shampoo is just a detergent. Like all detergents, it takes away oils and greases. Unlike your average kitchen dishwashing detergent, however, it’s very “gentle” — which means it has been severely diluted and therefore, on a cost per fluid ounce / milliliter basis, it outpaces Biden-priced gasoline. This is particularly true if you buy the “premium” brands (e.g. with French names).
Technically, you could use simple bodywash (also expensive) or even a bar soap like Zest to wash your hair, although it’s a little harsh if your hair is normally thin and fragile. However (and this leads into our sub-topic), what really counts, if you care for your hair at all, is not the detergent you use but the conditioner.
This is way more important than your shampoo, and a good conditioner will make your hair healthier than will some VO5-type budget conditioner — although, as with all things, budget conditioners work extremely well for some people because their hair responds to it better than to others, even expensive ones.
The more aggressive / cheaper your shampoo, the more money you’ll have to spend on conditioner.
So what do I use? The cheapest shampoo (generally to be found on the bottom shelf at Kroger, with the lowest cost per ounce) and a mid-range conditioner like Pantene Pro-V. But I have thick, healthy and wavy (not curly) hair, and I never use a blowdryer. Also, I wash my hair about every other day, and use conditioner once a week only.
YMMV.
Addendum: if you’re bald or wear your hair in a don’t-care buzz cut, you are obviously disqualified from commenting on this section, in that your opinions are like those of a cave-dwelling hermit about TV shows, or John Kerry about guns.