With the current special, there are over 500 games discounted. But I'm not interested in sorting by price, but by relative savings. How can I browse the titles sorted that way?
Since steamgamesales (mentioned in the other answer) appears to be down I found steamdb.info which allows you to search by all sort of criteria and appears to track historical prices, so you know how current discounts rank relative to previously-offered discounts.
The best way I've found is to go to steamgamesales and sort by "Savings". I'm not sure how often it updates, but it looks up to date. This site also includes sales from other websites besides Steam which provide Steam keys.