Some of the particularly good deals:
40% off The Witcher III
40% off Civ VI
82% off Borderlands “Take Over Your Life” Bundle (Borderlands + some DLC, Borderlands 2, Borderlands Pre-Sequel, Tales from the Borderlands)
50% off GTAV
91% off Valve Complete Pack (All Counter-Strike games, all Half Life games, L4D, L4D2, Portal, Portal 2, TF Classic, plus some smaller Valve titles)
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