I sold protein back in the day. My own brand. It's real simple.
1. Protein powders are mainly whey protein based. There are some veggie/plant and beef versions but the majority are whey.
2. Real protein powders (not be confused by gainers, blends etc) come in the concentrate versions or isolate versions
3. Said concentrates are 80-90% protein. Isolates are 90%+ protein.
We can debate the processing used to make a powder and whether or not the cows that produced the milk were grass fed or not but the differences are tiny. You can, as an ingredient, a digestive enzyme etc but again this wont make that much of an odds either way.
So you get into what's been added: flavours, sweeteners etc. Avoid if you can. If you MUST have these use naturally sourced ones. Some of the better ones have freeze dried fruit bits and the like. But the more additives they have the worse it gets. You can see the percentage of protein go down as the taste goes up. And some users will have stomach issues due to allergic reactions to these extras.
My preference is for an unflavoured concentrate. Isolate is better but the lack of carbs and fat mean it's more tasteless.