The best cat scratchers are the ones that your cat will use. But beyond that, some are better than others, i.e. more sturdy, longer lasting, etc. Here, I'll show you which ones are best according to what I have observed with our cats.
The most common cat scratchers that I have throughout our house are the corrugated cardboard styles. The preference for our kitties are the larger ones, that they can do multiple things with, like laying on them too.
The flat ones and wavy ones also work well. Our cats do seem to prefer the wider ones.
We do have a narrow one that hangs from a door knob. One cat likes this one, the other doesn't. The hanging style does make some noise when it is being used, so I added pieces of weather stripping to the back. It helps to reduce the banging sound against the door.
Both of our cats like scratching both horizontal and vertical surfaces. I have multiple types of scratchers around to give them a choice, and also the opportunity for them to use different muscles.
Our tortoiseshell cat prefers the cardboard vertical hanger above. Our tuxedo cat prefers the vertical sisal scratching post. After a couple of times of hearing the cardboard bang the door, that was enough of that one for her.
So overall, my recommendation is to observe your cat(s) to note any preferences, and to offer some variety.
The larger ones will probably cost more up front, but they last longer, so they can cost less in the long run. I will probably have to replace the larger cardboard styles from time to time (even after turning them over.) However, the sisal covered post has proven to be very long lasting and was well worth the cost.