I always buy a couple at the steam sale and then never play them. The only single player game I've played since Portal II was XCOM, which is fantastic btw. I love it when they actually make a game hard to beat.
Other than that it was just SC2 for me since SC2 came out and Dota2 since Dota2 came out. SC2 is just about as good as it gets for RTS and Dota2 is as good as it gets for dota. Both mainly cater to the autistic/ocd crowd. If you are autistic and you're not playing Dota2 right now, I don't know what you're doing.