The problem isn't the belt ( If your tensioner is working . ) , it is the flat pulleys becoming glazed.

A new belt will stop the noise for a bit but once the back polishes a bit the noise will return.

The fix is to roughen up any flat pulleys with 80 grit or so. This allows the belt to be gripped rather than sliding across the surface. When the engine is running, the belt tends to walk across the pulley causing the squeak.

I've also found that the part number sticker can be slippery enough to cause squeak, sanding this a bit makes the noise go away.

Also, never ever use belt dressing. It goes on sticky but soon drys to a slick , polished nearly impossible to remove mess that will increase noise.