I was hooked after about, oh, one episode. I COULD NOT STOP watching season one. I loved loved loved Rose, and really liked the Ninth Doctor, Christopher Eccleston (who was that invisible guy on Heroes!). I was upset that Eccleston was leaving because I did like him oh so much. But that lasted about five minutes (well, once during the Christmas special the Doctor actually starts doing stuff). And so began my full-blown obsession with David Tennant. And Doctor Who in general. I loved the first season but was completely and irrevocably obsessed by the second one. I cried, hard, at the end of the second season. I loved Rose Tyler. I miss her. The third season Christmas special was ok but I don't like Donna (so... err... hopefully she doesn't suck so bad for the 4th season). I liked Martha, though.
Ok, so a more specific season-by-season review.
Season 1: Best episodes were "The Empty Child" and "The Doctor Dances," the Steven Moffat-penned two-part episode. I have a theory about the dancing metaphor but anyway, ask me about that later if you care. "Dalek," "Father's Day," and "The End of the World" were also good. In addition, I liked the finale, "Bad Wolf" and "The Parting of the Ways." None of the episodes were particularly bad. I do like Captain Jack Harkness, too.
Season 2: Definitely my favorite season so far. David Tennant, as mentioned, is absolutely brilliant as the Doctor. The Christmas special was great, "The Girl in the Fireplace" was marvelous (Steven Moffat - see the trend?) as was the two-part "The Impossible Planet"/"The Satan Pit." I loved the introduction of K-9 in "School Reunion" and our glimpse of Sarah Jane Smith. The two-part Cybermen episodes were quite good as well. The finale was heartwrenching and great. So basically, no duds. However I must say in the first season and the beginning of the second I hated Mickey Smith. I wanted him to go away. He finally gets less annoying in the second season, though, and by the end of it he's alright by me.
Season 3: Rather hit or miss. I do like Martha, she's pretty much great. But there are definitely duds in this season (notably "The Lazarus Experiment," which was just bad, and the two-part Dalek episodes, which were amusing but not great). "Blink" is by far the best episode in the season. I also enjoyed "The Shakespeare Code," "Gridlock," and "42". The final trio of episodes is good too - especially the last one. I like that they brought back Jack Harkness, and that they hinted that he might be the Face of Boe...
I am looking forward to getting my season four in the mail! There's a Pompeii episode, which will be delightful since I'm a Classics major. hehe.
Doctor Who overall rating: A++++++
Season One: A
Season Two: A+
Season Three: B+