Quality in save for web

For photography, 100 and Optimized is the best.  80-100 would be good.

The lower you set the quality, the more picture data you throw out to reduce the image's space on your drive. That loss is permanent and gets worse if you resave. Jpeg is called a "final delivery format" for that reason. If disk space is no object, go with 100.

Now Save for web allows you to interactively preview what you see at the different quality levels. You may not notice at 100%, but blow it up to 300% and you can see edge artifacts and noticeable degrading in pure red areas at lower levels...about below 80. Jpeg is called "lossy compression" for that reason and is not meant for graphics and cartoons with sharp lines and pure colors. PNG is better.

You also see what amount of disk space the image will take up at lower settings, and you can make the decision as to the best compromise between quality and space in case you have to host the image on a site with data caps. Sometimes the site will even do that for you.

So there is no one number for everything. Use your eyes to see how much quality you can throw out if space is a problem. If not, 100 is best. I keep it there myself. If I lose the master copies (keep backups), at least it is the best quality.