Depends. I know that Final Draft comes with a number of templates that different television shows prefer for formatting. If you are wanting to write a spec based on an existing show, I would suggest FD, but I am a FD loyalist, its all I've ever worked with.

I vote for Final Draft. It has templates for any kind of film or show you want to write for. It also works with you as you type instead of taking you out of your normal writing mode to worry about formatting. It's also pretty much the industry standard and will work on both Mac and PC.