The first challenge is to write a drama spec, something in which i'm not qualified to do as I've never tried to write drama before. However, writing is about opening new doors and trying new things. So let's jump in. As of this year, there are a large number of TV Dramas, but how many of them are something you should spec? Let's take a look at a list.
The Americans, Hannibal, Sleepy Hollow, Elementary, Arrow. The Following. Bates Motel
These are current TV Dramas that are entering or are in their second season. There's no guarantee any of these will get picked up for another season, but they have a better chance then most. Why these rather then a more established show like Game of Thrones, House of Cards, American Horror Story? Because I'm looking for shows that won't cause you to have an aneurism trying to shoehorn in a storyline into an already complicated plot. Also note there are several other shows out there that are in their third season that are also great to spec, but might not be as eagerly read by someone.
This next list are shows that are still in their first season. There's no guarantee on these shows and some of them might not be up your alley, but it's worth keeping on eye on if summer rolls around and you need something to work on. Everything tagged with ** are shows that have not aired yet.
Almost Human, Marvel's Agents of SHIELD, Chicago PD, The Tomorrow People, Masters of Sex, Star-Crossed **, Resurrection **, Believe **, Crisis **, The 100 **, Extant **
The last piece here is that you need to set yourself apart. Pick something daring, something that's a little out of left field. Don't make it your only project, but if this is your second spec, try something a little out there.
Don't be afraid to fail. Your first few scripts are going to be bad. Not just, "Oh, I could have done that a bit better", but more "I have locked it in a closet rigged with dynamite" bad. You never want them to see the light of day. But recognizing bad writing is as important as being able to write well.
My first job in Los Angeles was as a reader for a development company. That meant spending hours doing what amounted to book reports on TV pilots and Screenplays that the company had access too and writing a short 2 page breakdown of the story and my thoughts on it. 90% of submissions were terrible. Those lucky 10% got my blessing and I went to bat for them in the room. That's what this whole thing is about, being that 10% and finding someone to champion your script.
So get writing.