I have fallen in love with plain, unflavored, cold processed (meaning filtered rather than cooked) whey protein isolate. It dissolves easily and quickly in most liquids; has a very mild, milky flavor; andnd you can add as much or as little of any sweetener you like.
Cold process whey protein also doesn't clump unless the liquid you add it to is too hot. If it's too hot to drink, it could clump. So if you mind clumps, let hot liquids cool before stirring in the protein powder).
There are other names for the cold process:
Undenatured (because heating denatures the proteins)
To make matters more confusing, some makers do not use any of these terms. Quick-mixing or quick-dissolving might be cold-process and might not. Some reference a "secret" formula or process that results in an better tasting and mixing protein. The "secret" really isn't secret, it's just cold-processing.
If you can find a local cheesemaker, you can often get a deal (unfortunately if you need it shipped, it's usually cheaper to find a national brand online, such as Syntrax Medical).