Really? Favorite engergy type? People actually have a favorite energy type?
3e (and 3.5) reduced all of the myriad idiosyncracies of energy damage to five: fire, cold, electricity, acid and sonic. And then, of course, there's the bizarrely labeled concepts of positive and negative energy.
How in the world do you have a favorite among those? Negative energy, while saddled with the stupidest name, is conceptually probably the most interesting. As a life-draining force, and the source of all undead, it's got an appropriate fantasy feel to it. I don't think the Negative Energy Plane is any better; but the Plane of Shadows, which is kinda like a more interesting Negative Energy Plane Lite is one of my favorites. The 4e book about the Shadowfell and Gloomwrought, the city therein, was one of the better bits of setting development 4e saw, too. Although, it wasn't specifically negative energy.
Still, that's my choice, and how I'd prefer to make it play a role in any of my campaigns.