Valve Complete Pack is very cheap. it contains every game Valve have ever made including the Portal series, the Half Life series and CS:GO. Also if you play GMod it’s a pretty useful thing to have.
Cuphead is pretty cheap normally but it’s 15% off currently. I haven’t played it myself but it’s apparently an amazing (and challenging) game.
South Park: The Stick of Truth is 75% off which I’d highly recommend. It’s made by Obsidian (the developers of Fallout New Vegas) and it is genuinely one of the best RPGs recently. It also doesn’t require UPlay unlike most Ubisoft games. Steer clear of the sequel which was not made by Obsidian.
And now a few short recommendations: L.A. Noire is pretty cheap and apparently still holds up very well. Grim Fandango Remastered is extremely cheap and extremely good if you like puzzle games Full Throttle Remastered is also fairly cheap and extremely good if you like adventure games
Sega’s 5th Annual Make War Not Love is starting in a couple days on Valentine’s. If you’re a fan of strategy games many of their series will be having sales, with bonuses for the main 4 games if they reached enough collective hours played by the community over the event. Or you can just sign up to grab Streets of Rage II and Revenge of Shinobi for free, I won’t judge.
Europa Universalis IV is 75% off and its expansions/DLC are 50% off on Steam until September 10th, the same going for Crusader Kings II and its expansions/DLC. (This excludes EUIV’s just-released Dharma expansion.)
A bunch of their other games/DLC are also on sale on Steam here.