Obviously this question is going to be asked at some point so I might as well be the obvious one and ask it...
I am assuming that a lot of people out there will rightly go with Genesis Of The Daleks and City Of Death. (Amusing side note, predictive text badly wants that first story to be renamed Genesis Of The Faeries). The first has Davros and the second is one of Douglas Adams' finest hours.
So I'll go for my 'official' vote The Deadly Assassin . Seeing Tom Baker on the back foot is always riveting. Watching him having to play out everything by himself is just so watchable, right from him putting a dummy of himself of him smoking a hookah to distract the guards to basically drowning to death in the matrix.
Anyone else? I had to think hard for a favourite 4th after discounting my usual defaults of Genesis and City, but that's not saying they're off limits for everyone!
The Creature from the Pit. It was the first full Doctor Who story I ever watched. I was probably 8 or 9 and they used to broadcast on the local PBS station after 10PM on Saturdays. It’s not a great story or episode. But the charm and camp of “green garbage bags” monster had me hooked.