You don't have much time to think; Croteam's games are on sale for only few more hours.
Titles from Serious Sam Series are 90% off.
Also, Talos Principle and it's DLCs are 66% off and Road to Gehenna is 25% off.
http://store.steampowered.com/search/?snr=1_4_4__12&term=croteam