(I swear I had this all written yesterday like it should have been, but my evening was weird so I forgot to post but HERE IT IS OMG LOOK)
This... well, this is a tough one. I have tons of favorite shows ever, and it all depends on what mood I'm in. I've answered this question before with Gilmore Girls, Friends, Scrubs, and Six Feet Under, and those answers have all been right at one time or another. However, if I'm considering my TV watching "career" as a whole, if I really look back at the show that's affected me most, that grabbed onto my heart and brain and eyeballs and didn't let go, that was there when I needed it... there's only one show that fits that description.
I didn't watch Buffy when it first aired, and while I regret that decision, I don't think this show would've meant as much to me as it does now. I watched the first season on a whim, and I totally loved it. I'd been wanting to ask for other seasons of the show for my birthday... and the day before said birthday, my grandfather passed away. Hubs had already gotten me the first 2 seasons as presents, so whenever I needed to separate myself from the family and distract myself from the grief, we hid away and watched Buffy kick some ass. In the weeks that followed, I basically devoured the show, hiding away in my bedroom and watching episode after episode after episode.
So, this sounds totally unhealthy, and I agree it probably wasn't the best way to spend my time. However, at that time in my life, it was exactly what I needed. Joss Whedon, you are a genius and I am so grateful for you. Thank you for this show, and for all the shows that have followed. Also the movies. And the comics. Dang, I am such a Whedonite.