Haven't decided between: Automation, BeamNG, WWE 2K16, or the bits of Crusader Kings II DLC I have yet to pick up.
If Automation's sale gets better (or I get another Steam Wallet card), then I'm picking it up without hesitation.
Update: Went with the CK II DLC. Picked up Conclave and Reaper's Due (with associated content packs) plus the Viking Metal pack. All that's left now is the Full Plate Metal music set.