I have a theory, the current doctor when you started watching is your favorite doctor. Would this apply to you?
I started watching Dr who when the reboot was 3 seasons in, I think David Tennant was the greatest doctor. A few years later I finally got wife to give it a shot as I started a fresh rewatch in preparation for Matt Smith's second season, she was ok with Eccelston and Tennant but Matt Smith is far and away her favorite doctor. Does this pattern hold true for any other Dr fans?
It's hard to pick a favorite. Technically the first Doctor I watched was Eccelston, but I don't really count him as first since I was catching bits and pieces of an episode while sick with the flu or something. Tennant was my first, proper introduction to Doctor Who and I worked back from there. I still enjoy Tennant's Doctor a lot, but I wouldn't necessarily say he's my favorite. If I had to pick one I'd have to go with Capaldi. Maybe I just liked the novelty of an "older" Doctor after Tennant and Smith, but he did have some great moments during his run.
I also started with Eccleston and have Capaldi as my favorite. I found his grumpy Doctor a good change up and Clara Oswald was one of my favorite companions, but I think the best show writing happened during the Tennant/Smith eras.