Bought a nice scratcher and your cat ignored it completely? It is probably the wrong type. Cats have a clear preference between scratching upright or lying down, and getting that right solves half the couch problem.
Vertical scratchers
These are the posts and towers, where the cat stands on its back legs and stretches up. Most cats like them, because it mimics scratching a tree trunk. The golden rule is height: tall enough for a full stretch, and sturdy enough not to wobble. A post that shakes scares the cat off.
Horizontal scratchers
These are the pads and ramps on the floor, often made of cardboard. Some cats prefer to scratch lying down or on an angle, and those cats will love it. They tend to be cheaper and take up less space, so they are great for testing before you invest in a tower.
How to find out what yours prefers
Look at where it already scratches. If it attacks the side of the couch standing up, go vertical. If it rakes the rug lying down, go horizontal. When in doubt, offer both for a week and see which gets more use. It is cheap to find out and saves you from a tower it will ignore.
What about material?
Sisal is usually the favorite, because it has the texture cats love to shred. Pressed cardboard also works and is cheap. Plain carpet works less well and can confuse the cat, which may start scratching your actual rug by association.
The short version
There is no absolute best, just the best for your cat. A tall, sturdy vertical suits most, while a sisal or cardboard horizontal suits floor-scratchers. Find the preference and you fix the couch problem for good.
Frequently asked questions
My cat ignores the scratcher. Did I buy the wrong one?
It might just be in the wrong spot. A scratcher hidden in a corner is useless; cats scratch where they mark territory, near where they live and sleep. Move it next to the couch or a walkway.
Sisal, cardboard, or carpet?
Sisal lasts longest and most cats prefer it. Cardboard is cheap and many cats love it, but it sheds crumbs. Carpet confuses them (it feels like your rug). When in doubt, sisal.
Do I need both vertical and horizontal?
If the budget allows, yes. Many cats use both at different moments: vertical for the big wake-up stretch, horizontal mid-play.