So what's the best way to pick a prop?

Depends on what you want to do. If the prop is a major player, quit reading right now and find an experienced modeler in whatever that kind of flying is. He'll save you lots of time and tears.

But for most people, start out with one of the engine mfg's recommended props. Start with one that has a larger diameter, not a smaller, for whatever you want to do. Want to 3D, then it'll be a large diameter prop to begin with. Going plyon racing? Find an experienced plyon guy and quit reading this mess. Otherwise, pick a larger diameter. And get a couple with different pitches. Best bet is to stick with one brand in these first steps.

Larger diameters often idle better. More mass in motion helps there. They also are more apt to give more uniform performance. They engine winds up having more constant load. That is, if the prop allows rpm that isn't on the low end of the engine's torque curve. Most mfg's won't recommend a prop that isn't safely on that curve. More mass and steadier load often restrict the engines ability to accelerate. But that won't matter much unless the pitch is low and you're looking for acceleration. If the pitch is one of the lower numbers, any prop/engine will usually be able to move up and down the rpm range quicker. That usually does give better acceleration and deceleration. Pattern flyers like to have deceleration, btw.

Test a number of different pitches and get used to the plane with that diameter. Now try out another brand of props. Try the same diameter and to save a bit of time and money, you can just buy one. Same pitch as the favorite from the 1st brand. Doing that might save you buying some worthless props. If the new brand prop's pitch is say a 7 and it acts like the 1st brand's 6, that info will let you choose the next test prop's pitch.