First: Why write a spec? The traditional reason is that it shows a showrunner that you can write in the voice of another person, since if you get hired on a show you’ll have to write in the showrunner’s voice. The modern reason is you want to enter the summer writing contests. And anyway, it’s a good idea to have one in your portfolio. Preferably one each year. Just in case.
So now you’re convinced. Which show? I recommend starting at the WB Writers’ Workshop Application page. If you scroll down just a tad, you’ll see drop down menus next to Comedy and Drama. Pick your category and scroll through the menu. Write down all the shows on their list that you watch and like. These are all shows that have been on the air for at least a full year (since so many first-year shows get canceled or are still finding their footing, it’s not a good idea to spec them anyway) and that there’s a reasonable possibility your reader has watched. And since you’re submitting to the contests anyway, might as well be sure the show will qualify.
Next, check out the nominees for the most recent or current Emmys, Golden Globes, and WGA Awards. (Note: if you just focus on winners, you’ll narrow your field too much. Check out all the nominees.) You’re looking for the best series and best writing categories. If any of those shows are on your list, put a little star next to them.
Finally, research! Look up ratings, and what people have been saying. Are any of the shows you’re looking at not doing well? Is anyone using the phrase “on the bubble?” If so, cross that show off your list. Although you can technically use a spec script after the show has been canceled, it’s not a good idea. And you don’t want your script to be dated the moment it’s finished. Make sure it’s a show that will still be around next year.
And there’s your list. As you look at it, there’s probably already something you want to write. If not, just keep it in mind while you think about story ideas. I had been thinking I would write an Elementary this winter (it hits the sweet spot of being fairly new, but not too new, but definitely coming back, plus a procedural which is so much easier) and then I got an idea for a Good Wife, so there you go. Good luck!