Single player games mainly because I like to do things at my own pace (which can be a faster pace than some) and where I have full control on what I want to do without behind held back.
I'm at a point in my life where I no longer have the patience to wait on someone - back when I did play MP/online games, always felt like being held back by less than stellar team mates.
For an example I played a MMO with a guild - me and a friend farmed gear ready for PvM and PvP in a week while most people in the guild we were in probably got one piece of armor but took them a week just to farm that one piece (using inefficient methods, playing poorly, etc).
For MP games, I tend to get ones that have AI controlled (Left 4 Dead).