My first doctor was Tom Baker, then watched a lot of the others. But Tom Baker remained my favorite through the years, but the moment this next guy stepped in, he won first place from his first episode as the Doctor:

Now, David Tennant has left the role, and from March 13 we'll have a new Doctor: Matt Smith. I'll reserve my judgment until I see all of the new season.
Best episodes of the new series:
From Season 1:
The End of the World
Father's Day
Dalek
The Emty Child/The Doctor Dances
Bad Wolf/The Parting of the Ways
Season 2:
The Christmas Invasion (because we get introduced to David Tennant)
School Reunion (Rose & Sarah Jane!)
The Girl in the Fireplace
Rise of the Cybermen/The Age of Steel
The Impossible Planet/The Satan Pit
Army of Ghosts/Doomsday
Season 3:
42
Human Nature/The Family of Blood
Blink (one of the top 5 episodes ever)
Utopia/Sound of Drums/Last of the Time Lords
Season 4:
The Sontaran Stratagem/The Poison Sky
Silence in the Library/Forest of the Dead
Midnight
Turn Left
Stolen Earth/Journey's End
Waters of Mars
What are your favorite episodes? And which Doctor is your favorite?