A close second is "Mummy on the Orient Express".
But I'd have to go with "Flatline". It just got so much right.
The monsters were inventive, well realized with the visuals, and I like how it touches on the idea that they are unwittingly hurting people, only to subvert it, which led to one of the best Doctor speeches of the whole show, which really cemented Capaldi in the role for me.
The gimmick of having the TARDIS shrink was not only an inventive way to get around a Doctor-light episode, but led to so much fun visual humour. Plus it gave way to one of Clara's better episodes; she was really likable in this one.
On top of all that, it touches on the arcing plot just enough without being intrusive, has fun supporting characters, its just dark enough, but most importantly of all, its fun fun, fun-fun Fun!!! And that's what really matters.