The 9th doctor is my absolute favorite doctor for a lot of reasons. I might be biased since he's the first doctor I had the honor of watching BUT STILL.
Firstly, this is the first time we meet Rose, an absolutely lovely personality, just the right amount of oomph and kindness. The 9th doctor just adores everything and everyone, even when he hates them, which makes them a perfect match.
Christopher Eccleston does an amazing job at showing the switches in the Doctors emotion. We see him giggly and ecstatic and the next second, serious and haunted. He just carries a feeling of history with him.
David Tennant did an absolutely brilliant job at bringing that to life as well, even adds to it when it comes to Rose.
But I just have a soft spot when it comes to the 9th doctor, he's severely under appreciated.