PC: The greatest and most convenient platform for local multiplayer.
An argument could actually be made for this in terms of development, but not for end-users. The general set-up for most PCs makes local multiplayer too much of a hassle. The one modern concession to this would be connecting laptops to living room screens.
The biggest problem with PC local multiplayer is the controller dilemma. One of the best developments for local multiplayer in the past decade has been wireless controllers. Having a controller interface that requires no wires makes local multiplayer much easier to set up and deal with. The problem is that the PC platform STILL doesn't have any standardized controller, and the closest it has gotten hasn't been wireless. (the wired 360 controller was much more standard for the PC as opposed to the wireless model)
Recent developments might change this. The Dualshock 4 apparently plays very well with most PCs. (with bluetooth adapters, at least) And the Steam Controller is being developed with PC in mind.
I don't think there is really a controller dilemma anymore. Like you said, the Dualshock and the Steam controllers work with PCs. I have a wireless Xbox controller dongle and I used that to play multiplayer with my cousins over the holidays. It was easy because they brought their controllers that they use with their console and it worked perfectly. I think the biggest problem for local PC multiplayer is the mainstream perception that these games don't exist on PC, and the fact that most people would rather play these games on a TV than a computer monitor, and they either don't want to or don't know that their computer can be displayed with a TV.
