Steam may be losing some luster in the digital game distribution arena with the quantity and quality of heavy players that have appeared since its inception - I'm looking at GOG and the EPIC Games Store especially. However, Steam continues to be the service that offers the largest library of purchasable titles. As such, if you don't have a backlog of games to play already (don't lie, you do have them, but we won't judge if you buy some more), now is the time to make the jump.
The biggest discount percentage I've seen, so far, is that for Rise of the Tomb Raider, which cuts a total of 85% from its listing price. Discounts range from 20% through to that (apparent) ceiling, with franchises such as the Dark Souls Series (Dark Souls 3 for $15 (75% off), Dark Souls 2 for $9.99 (75% off), and Dark Souls Remastered going for $23.99 (40% off). The Dishonored franchise also sees massive cuts from their list prices, at 70% for the original and 75% for Dishonored 2 and its Death of the Outsider expansion (how I love these games). The Master Chief Collection, however, isn't on sale - it's still in pre order, you know how it is. But maybe one of these days you'll get that one (though it already is a steal at its launch price for the quality of games you're getting). Go on, feast your eyes and murder your wallet. We're cheering for you.